There is a consistent effort to study and evaluate the impact of a health policy on different stakeholders. New York State passed Timothy’s law to improve the mental disparity and provide more support to patients with certain mental diseases and not eligible for Medicaid and Medicare in Jan 2007. This law requires many insurance companies to provide coverage on most of the mental illness. The law became permanent later in 2009. Although there was an evaluation of this policy on the cost side by the New York State Insurance Department in 2009, there is lack of studies on its impact on patients’ health outcomes, such as care utilization. In this study, we will use a hybrid of propensity score method and Difference-in-Differences method to evaluate the causal effect of Timothy’s Law on inpatient visits of mental patients. The Healthcare Cost and Utilization Project (HCUP) state inpatient data between 2006 and 2007 was used. Considering the comparability in characteristics economically and politically, California was chosen to be the control group at the same time frame. In this presentation, we will introduce the motivation, the analysis, and the result of this study.