It is hard to negate the effects of ocean waves on a synthetic aperture sonar that has to operate in the ocean, and many efforts against the ocean waves had been taken. One of the effort is motion compensation by calculating phase errors with computer. But the synthetic aperture algorithm with motion compensation is heavy, and remains difficult to be executed on real time. Thus, modified synthetic aperture algorithm is proposed in this paper. The algorithm, which is named 1.5-Dimensional algorithm, is in between 1-Dimensional and 2-Dimensional algorithm. The algorithm synthesizes an virtual aperture faster than the others, and has no phase error theoretically. The algorithm had been executed with raw sonar data acquired in a sea trial experimentally, and higher quality sonar images are generated successufully.