In 1986, a joint research between China and Japan started in order to conserve cultural relics in China. The authors, members of the research team, studied the climate of the Mogao caves at Dunhuang. The temperature, humidity, and sunshine were measured at cave Number 194. This cave is located in the third row, that is, on the upper position of a cliff. The cave, which is a rather small one among the Mogao caves, is about 4 m wide, 7 m deep, and 3 m high. The measurement started in March, 1988. The temperature and humidity were recorded every three hours, eight times a day, in a data logger. The analysis of the climate data from March to June of 1988 revealed that the daily change was much smaller in the cave than outside. The paintings have been well conserved due, perhaps, to this stable and dry environment. But a sudden and large increase of humidity occurred within the cave during nights in June. The authors were concerned about this phenomenon from the point of view of conservation. The increase was probably caused by rain. Even though the Mogao caves are situated in a dry and desert region, heavy rainfalls are occasionally observed. Sometimes precipitation during such heavy rain reaches nearly the annual amount of rainfall. In such a case, the paintings will certainly suffer great damage from the invasion of moisture. Examination of the paintings of the cave proved that many fragments had flaked off in the past due to the efflorescence of soluble salts.