Abstract: Abstract: Here, a computer vision on-line detection system was applied to Shiitake mushrooms (Lentinus edodes) during the hot air drying with the process control of Relative Humidity (RH). Four RH controls were used, including continuous fanning, constant target RH throughout the whole process, reducing the RH at different stages, and rapidly reducing the RH at the later stage. An image processing using region segmentation was selected to on-line measure the Ratio of Wrinkled Surface Area (RWSA), in order to separate the wrinkled areas from the non-wrinkled areas on the surface of shiitake mushrooms. The roundness in the image was used to represent the shape of the edge of shiitake mushrooms. The Cell Aspect Ratio (CAR) was used to quantify the degree of deformation in a cell of shiitake mushrooms, which was calculated by the pixel length of cells along the radial and vertical direction in the microscopic image. The appearance features (contrast and energy) of shiitake mushrooms were obtained by the Gray Level Co-occurrence Matrix (GLCM). A measurement was made for the changes of shrinkage, roundness, RWSA, and visible texture (contrast and energy) characteristics during the drying process of shiitake mushrooms, and thereby to establish the relationship among appearance qualities, Rehydration Ratio (RR), and microstructure of shiitake mushrooms. The results showed that the RH had significant effects (P < 0.05) on the appearance qualities (shrinkage, roundness, RWSA, contrast, and energy), the microstructure, and rehydration ratio of shiitake mushrooms. Compared with the continuous fanning, higher RH maintained in the drying room at the initial stage of drying, corresponding to higher internal temperature of shiitake mushrooms, while longer drying time. The higher the initial RH or the longer the RH holding time was, the lower the drying rate and surface hardening degree were. Therefore, the shiitake mushrooms were more sensitive to shrinkage and deformation. The roundness changes of shiitake mushrooms corresponded to the change of shrinkages, where the higher the shiitake mushrooms shrinkage was, the lower the roundness was, indicating the more irregular shape of shiitake mushrooms. The slow rate of surface hardening led to more wrinkles on the surface of shiitake mushrooms, and lower color contrast distribution in the neighborhood pixels on the images, indicating that the color of non-wrinkled area was close to that of wrinkled area, and the whole surface color of shiitake mushroom was darker. Meanwhile, the cell aspect ratio of shiitake mushrooms cells was larger, indicating that the deformation of cells was more serious. At the same time, the small cell pore resulted in very low rehydration ratio of shiitake mushrooms. Compared with the whole process groups, the drying time, and shrinkage were significantly (P < 0.05) reduced, the roundness improved significantly (P < 0.05), the RWSAs reduced (P > 0.05), and the surface color of shiitake mushrooms lighter (P < 0.05) in the optimized groups by turning on the dehumidification fan, when the moisture evaporation was not enough to maintain the set RH range in the drying room. The reason can be the increased differential partial pressure between the air and the surface of shiitake mushrooms, resulting in an accelerated drying rate, while rapidly hardened and shaped surface of shiitake mushrooms. However, the deformation degree of shiitake mushroom cells was not significantly (P > 0.05) reduced, and there was no increase in the erehydration ratio. A positive correlation was observed between the time, when the RWSA reached the maximum and the rehydration ratio of shiitake mushrooms (r = 0.88). High relative humidity had a negative effect on the flavor components of shiitake mushrooms. The best drying was continuous fanning, according to the drying time, appearance quality, sensory quality, and composition of dried shiitake mushrooms.