Moving particle semi-implicit (MPS) method is one of the particle methods, which can be used to analyze incompressible free surface flow without surface tracking by a mesh or a scalar quantity. However, MPS causes unphysical numerical oscillation of pressure with high frequencies. We proposed a new formulation for the source term of Poisson equation of pressure. The proposed source term consists of three parts, one main part and two error-compensating parts. With proper selection of the coefficients for the error-compensating parts, we can suppress the unphysical pressure oscillation. Smoother pressure distributions are obtained in hydrostatic pressure and dam break problems. Copyright © 2010 John Wiley & Sons, Ltd.