Water content and changes in apple ring thickness were measured during the drying process. The initial thickness of apple rings amounted to 5, 7 and 10 mm. The apples were dried in the temperature of 55°C, at the drying air flow velocity of 0,5 m·s^-1. On the basis of the conducted studies we found out a significant influence of dried apple fineness degree in the conditions of forced convection on the values of effective water diffusion coefficient. With the increase in thickness of dried rings the values of effective water diffusion coefficient grow. It was also noticed that in the initial period of drying, the values of water diffusion coefficient increase for each thickness of apple rings, despite the decrease of humidity of the dried material. It is after a certain time, amounting to ? _(5 mm) = 4,5·10³ s, ? _{(7 i 10 mm)} = 6,3·10³ s, respectively, for the ring thickness of L = 5 mm, 7 and 10 mm, that the decrease of humidity in the dried raw material is accompanied by the decrease of the value of this coefficient, continuing till the end of drying process, i.e. to the humidity of 16%.

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