Abstract: Based on the Seaflux dataset from 1998 to 2007, the seasonal variability and formation mechanism of diurnal warming of sea surface temperature (dSST) are revealed in the Bay of Bengal (BOB) and its adjacent area. The results show that dSST varies in annual cycle in the south equatorial area of 5.0°N. The annual cycle presents a single-peak structure and reaches its maxima (minima) from December to May (June to November). The semiannual signal occurs in the north of 5.0°N and presents a unique double-peak structure with lager amplitude in spring and autumn while small amplitude in summer and winter. In spring, the range of dSST is higher in the center of the bay and lower in the surroundings. While dSST is lower in the bay mouth it is higher in the bay center and equatorial region in autumn. In summer and winter the distribution of dSST represents a similar pattern of higher in the equatorial region and lower in the bay, but high value centers show a bit different in the northeast Sri Lanka and western boundary of the bay, respectively. Further analyses show that surface wind is vital over the entire area and determines the seasonal and spatial distribution of dSST. Solar shortwave radiation plays a significant role in the variability of dSST in the BOB, while its effect is weaker in the south of the BOB and most equatorial regions.