A method of GPS accuracy adjustment for an integrated INS/GPS navigation system which utilizes microelectro mechanical systems (MEMS) based inertial sensors to mitigate multipath problems arising when a vehicle is in an area with many high-rise buildings is disclosed. Even when GPS measurement output values are deviated from that expected, the INS/GPS navigation system uses as many measurement outliers as possible without discarding, unless they are obvious error values, thereby maintaining a practical level of accuracy. The measurement outliers typically occur when the integrated INS/GPS navigation system receives GPS signals which have transmitted through multipaths such as reflection by buildings, since the signal path lengths vary.