Based on the data of 33.44 GHz millimeter wave radar detection, combined with weather situation and automatic stations nearby, a sea fog from the night of 27 March to the morning of 28 March 2018 in the Ningbo coast is analyzed. The results show that it was mainly a radiation fog process, having abundant water vapor below 850 hPa, inversion temperature and high ground pressure, which provided favorable conditions for it. The sea fog developed from lower to upper with a maximum echo height of about 230 m and disappeared from top to bottom, and there was a wind convergence and calm zone being beneficial to the sea fog. The disappearance of the sea fog was more than 3 hours later than that on the land. The echo height and intensity both had a trend of double peaks, except for the generation and disappearance stages, and the occurrence time and causes were different. In addition to the generation stage, the reflectivity was inhomogeneous in the horizontal direction, and the lowlevel reflectivity was generally stronger than that of the higher layer. Compared with visibility stations， millimeter wave radar can detect the variation and vertical structure of sea fog more effectively; however, due to the small reflectance, sea fogs could not be detected in some areas.