Abstract: The long period crustal movement prior to earthquakes has been noticed for a long time. However, the problem of zerodrift of long period seismographs rendered the observations with such seismographs tentative or experimental in nature. Besides, when the underground gas spurts oat and sets free before earthquakes, the observation of such gas can be made only near it but not too far away. The microbaro-mic waves, such as infrasonic and gravity waves caused by the gas outspurts and long period ground movements prior to earthquakes may be detected at a distance, thus should be studied.Infrasound caused by seismic waves has been noticed long ago, but no report on infrasound before earthquakes has been made. After the 1976 Tangshan earthquake, from August 1976 to March 1977, the author observed microbaromic waves prior to aftershocks with microbarograph at Peking. More than ten of this kind of waves were observed. Its characteristics are as follows: the period is 40—80 seconds, sonic pressure 1—5μb(microbar), duration lasts a few minutes and it appeared 1—16 hours prior to the earthquakes.In this paper, a method of analysis for recognizing microbaromic waves prior to earthquakes is developed. Its possible mechanism and other phenomena have been discussed.