The purpose of this study is to use the direct and reflected signals of GPS to determine the patterns of water wave in the water flume, as well as to establish a practicable technique of measuring sea water level. The measurements of (1) water waves in the Super Tank and tranquil water level with steadily descending water level in the Mid-size wave flume of the Tainan Hydraulics Laboratory, National Cheng Kung University, and (2) sea water level in the Tainan An-Ping Harbor, were performed. In this research, an integrated GPS receiver that employed direct and reflected GPS signals for the measurement was introduced. Both RHCP and LHCP antennas were employed to simultaneously receive the L1 and L2 carrier phase of direct and reflected signals. After the data analysis of the GPS observation, the position of signal reflection and the water level of the wave were solved. After improving RGPS positioning procedure, the tranquil water level and the steadily descending of water level observation for one hour were performed, and the results of the RGPS, wave gauge, and staff meter were coincided within 0.5cm ∼ 1.0 cm. In the Super Tank, the 60cm amplitude with 5 second/cycle of regular wave was generated. The differences between the reflection heights of this GPS system and the record of wave gauge were almost identical within 90%. In addition, a 6-hour observation of sea water level in An-Pin Harbor indicated that results among the RGPS, tide gauge, and staff meter had only 1.0cm ∼ 3.0cm differences.