The interannual variability of the western North Pacific (WNP) summer monsoon is examined for the non-ENSO, ENSO developing and ENSO decaying years respectively. The ENSO developing (decaying) year is defined as the year before (after) the mature phase of ENSO, and the non-ENSO year is defined as the year that is neither the ENSO developing year nor the ENSO decaying year. A strong (weak) WNP summer monsoon tends to occur during the El Niņo (La Niņa) developing year and a weak (strong) WNP summer monsoon tends to occur during the El Niņo (La Niņa) decaying year. In all non-ENSO, ENSO developing and ENSO decaying years, the strong (weak) WNP summer monsoon is associated with the positive (negative) rainfall anomalies, cold (warm) sea surface temperature anomalies, warm (cold) upper tropospheric temperature anomalies, low (high) surface pressure anomalies and a low-level cyclonic (anticyclonic) circulation anomaly over the subtropical WNP. The 850 hPa wave train associated with the WNP and EA summer monsoons in the non-ENSO, ENSO developing and ENSO decaying years extends northward and suggests a possible teleconnection between the WNP summer monsoon and the North American climate. The wave train extended into the Southern Hemisphere in the non-ENSO and ENSO developing years implies a teleconnection between the WNP summer monsoon and the Australian winter climate. The anomalous WNP monsoon in the non-ENSO and ENSO developing years exists only in summer, while the anomalous WNP monsoon in the ENSO decaying year persists from the beginning of the year to the summer season. The anomalous WNP summer monsoon exhibits a strong ocean-atmosphere interaction, especially in the ENSO decaying year. This study suggests that the anomalous WNP summer monsoon in the non-ENSO year is associated with the variation of the meridional temperature gradient in the upper troposphere, while the anomalous WNP summer monsoon in the ENSO developing and decaying years is associated with ENSO related SST anomalies.