The Tonkin Gulf is a semi-enclosed bay in the South China Sea with a shallow shelf. Historic observations indicate that the coastal and outer water masses converge here, which results in a complex hydrographic situation. The thermal front in ocean is a good indicator for connection between different water masses and acts as an important topic in the oceanography. Here the 8-year (1991 - 1998) Pathfinder sea surface temperature (SST) data have been applied to investigate the seasonal and interannual variability of the thermal front in the Tonkin Gulf. This paper tries a new methodology to describe the gradient of temperature using a pairs of orthogonal vector components. When the variability of the thermal fronts in the Tonkin Gulf is concerned, the signs of SST's anomalies (SSTA), the deviations of surface wind and surface heat fluxes are individually accounted for criterions of the causes. In the research domain, the order that the thermal frontal intensity varies with seasons from strongest to weakest is spring, then summer, winter and fall. There is a significant relationship between the frontal intesity and SSTA, which indicates a trend that when SSTA is negative, the fronts are strong, and the absolute value of SSTA shows a positive correlation with the frontal intensity. In winter, when abnormal northerly appears over the bay the front is stronger, whereas the front is weaker when anbnormal southerly occurs.