We present a study of HII regions in M51 using the Hubble Space Telescope Advanced Camera for Surveys images taken as part of the Hubble Heritage Program. We have cataloged about 19600 HII regions in M51 with H{alpha} luminosity in the range of L=10^35.5^-10^39.0^erg/s. The H{alpha} luminosity function of HII regions (HII LF) in M51 is well represented by a double power law with its index {alpha}=-2.25+/-0.02 for the bright part and {alpha}=-1.42+/-0.01 for the faint part, separated at a break point L=10^37.1^erg/s. This break was not found in previous studies of M51 HII regions. Comparison with simulated HII LFs suggests that this break is caused by the transition of HII region ionizing sources, from low-mass clusters (with ~10^3^M_{sun}_, including several OB stars) to more massive clusters (including several tens of OB stars). The HII LFs with L<10^37.1^erg/s are found to have different slopes for different parts in M51: the HII LF for the interarm region is steeper than those for the arm and the nuclear regions. This observed difference in HII LFs can be explained by evolutionary effects: HII regions in the interarm region are relatively older than those in the other parts of M51.