This work is focused on investigation of internal structures of the southeast Asian summer monsoon (SEASM) using an empirical orthogonal function (EOF) or principal component analysis (PCA). Characteristic of the monsoon patterns can be extracted from the horizontal winds over southeast Asia. The seasonal climatological mean of the horizontal winds during the monsoon period June-September from 1971-2010 has been taken into account. The PC time series are closely correlated with variety of monsoon indices (MI), especially during 1979-2000, incorporated with explaining the monsoon decomposition for the spatial patterns upto four modes of significant. The interpretition of spatial patterns reveals the hidden prevailing flow patterns which can be linked to the intraseasonal and interannual variability of the summer monsoon effect on the rainfall anomalies.