Abstract: The environment of naturally ventilated sheep house was tested and analyzed at a sheep farm in Inner Mongolia in winter. The results show that the average indoor temperature and relative humidity of sheep house at night are -2.8～1.2 ℃ and 64.5%～73.5%, respectively. The indoor temperature and the relative humidity of sheep house are influenced by the outdoor temperature and the outdoor relative humidity. The indoor temperature is 9～12℃ higher than the outdoor one, and the temperature indoor fluctuates less than that outdoor. The air velocity, CO₂ and NH₃ concentration are suitable for sheep requirement. The results show that the indoor temperature and the relative humidity are obvious different when the sheep house orientation is different. The indoor temperature of houses facing to the south is 2～3℃ higher than that to the east. The indoor temperature of houses with the windows closed is 2～4℃ higher than that with the north windows opened. The naturally ventilated sheep house improves the environment situation to some degrees and alleviates the hazard of inclement weather to the sheep industry. But the house without any heating system can not meet the environment requests of lying lamb in winter.