Abstract: In view of the diagnosis of outofcontrol signals in multivariate statistical process control charts, a method of multivariate process diagnosis based on path diagram was proposed. Utilizing the quantitative causation among variables given by the path diagram, a simultaneous equation model could be obtained by establishing the linear regression equations for all variables independently. Then, the simultaneous equation model was applied to derive a uniform decomposition for T2 statistic. And the control limits for all decomposition terms were given as the standards of judging the root causes contributing to the outofcontrol signals. Simulation studies for a system consisting of three variables were carried out to demonstrate that the proposed method is reasonable.