Abstract: The characteristic features of westward propagating 40-day low-frequency oscillations over the equatorial region during the period of July-December 1981 are analyzed by using the band-pass filter and EOF composite technique. The results show that the crosequatorial 40-day temperature waves over the eastern Pacific are mainly responsible for the formation of the westward propagating modes. There are two main sources for these modes. The one is in the vicinity of 1500E at equator. The other is in the vicinity of 110°W at equator. The lowfrequency disturbances formed in these two places are different from each other.The results of vertically integrated heat source Q1 and moisture Q2 show that the Q1 heating centres propagate across the equator over eastern Pacific, which is in good agreement with the propagation of the cross-equatorial temperature waves. The Q2 heating centres seem to originate from around 150°E at equator and move northwestward.