The air temperature, humidity and pressure in Cave 108 of the Mogao Grottoes were monitored by twelve sensors set at different positions inside. The results show that the air temperature in the cave has obvious daily and long-term variations. The variation amplitude of the upper part of the cave is larger than that of the lower part. In summer, the temperature of the upper part is also higher than that of the lower part. The absolute humidity of the air inside is seriously affected by the environment outside the cave and has a daily variation as well. Regarding the distribution, the air humidity is higher in the upper and the front parts of the cave than in the lower and the rear parts. There exists a net water vapor input from outside, but not through the cave entrance. Similarly, affected by the environment outside, the air pressure also has a clear daily variation and is higher on the east side of the cave than on the west side. All of these results above indicate that the moisture and heat sources of Cave 108 exist near the upper part of the east side of the cave.