Indo-Pacific Warm Pool (IPWP) hosts the warmest water across the global ocean. However, the variance of the sea surface temperature (SST) over IPWP is significantly smaller than those of other regions. Hence the large amplitude marine heatwaves (MHWs) rarely occur over IPWP. But this does mean there is no serious MHWs over IPWP. Historical SST analysis reveals that the MHWs over the monsoonal ocean exhibit the strong phased-locking features. The most significant MHWs occurs during the pre-monsoon period just before the monsoon onset, especially over the central Bay of Bengal, between Indonesia and Australia, and southern and northern South China Sea . It is particularly noted that the notorious MHWs happen in the late monsoon onset years.  It is also interesting to note that much less MHWs occur in the central area of the Coral Trangle region. The unique phase-locking feature of MWHs associated with the monsoon seasonal cycle underlines the challenge of prediction, considering the fact that the monsoon onset is still poorly reproduced in the coupled global circulation models (GCMs).