There are three sources of error in gridded partial pressure of CO2 (pCO2), spatial variance, under-sampling variance, and measurement error. These sources of error in gridded pCO2 in the northern South China Sea were quantified using underway pCO2 data and model-derived pCO2 data. The seasonal signal of pCO2 was interpreted based on gridded pCO2 on 1/2°×1/2° grids. In general, summer has higher pCO2 than the other seasons in the northern South China Sea and winter has the lowest pCO2. This is consistent with previous studies. Greater uncertainty usually appears near the coast, mostly caused by greater spatial variance and a few caused by poor sampling coverage.