Antenna gain is an important parameter in weather radar equation and has a direct influence on accuracy of reflectivity measurement. This paper analyzes the feasibility of antenna gain measurement for weather radar using the sun as a signal source, and introduces relevant principles and methods. Then it measures antenna gain of a Cband mobile China new generation radar (CINRAD) by the sun and analyzes error sources as well as methods to improve measurement accuracy. Observation experiment shows that antenna gain measured using the sun is close to factory test result while measurement accuracy requires further improvement. Therefore, the sun can be used to monitor variation of antenna gain in the operational weather radar network regularly, which is helpful for identifying and diagnosing breakdowns in antenna pointing, feeder line loss and receiver in weather radar system as early as possible.