The use of synchrophasor data for observation and control is expected to enhance the operation and efficiency of the next generation of power transmission systems. The synchropha-sor measurement data is usually transferred over public domain networks such as the Internet, thereby making it susceptible to a number of attacks. This paper focuses on packet dropping or gray hole attacks on networks carrying synchrophasor data and develops a mechanism to detect such attacks. Our solutions is based on exploiting the patterns and correlation between packet delays and packet losses due to congestion in order to differentiate naturally occurring packet drops in the Internet from packet drops by gray hole attacks. The effectiveness of the proposed mechanism has been verified using simulations.