Abstract: Air circulation velocities were measured in three lumber drying kilns of the lateral fan type and were found to be nonuniform in different parts, but regular. The general pattern of the velocity is as follows: along the vertical axis from the top of the kiln to the bottom the circulation velocity is of a low value at the top, increases gradually downwards and then decreases to the minimum at the middle level of the vertical axis somewhere just above the rim of the fan. The air then flows back in the opposite direction and repeats the pattern described above.It is believed that the uneven air circulation would cause nonuniformity in the dryness of the lumber in the stacks, but on the contrary, analyses of the variance of the final MCs based on the results of superheated vapour drying experiments carried out in these three kilns at an average temperature of about 110℃ showed that the differences of the final MCs along the height, the width and the length of the stacks are statistically insignificant, which would mean that the unevenness of the circulation of air exercises little influence to the lumber at different positions of the stacks. Reasons for this phenomenon are assumed as follows: Firstly, the real circulation velocity of the superheated vapour might be much higher than the measured veloeity of the flowing air. Secondly, the great amount of vapour evaporated from the lumber might increase the circulation velocity during the process of superheated vapour drying. Thirdly, the vapour coming from the wetter lumber pieces might raise the EMC of the drier pieces so as to prevent the latter from drying faster.