INTRODUCTION: Medical marijuana is legal in 25 states and recreational marijuana is legal in 5 states in 2016. AIM: This study investigates if and how marijuana legalization expansion affects the use of marijuana and how medical and recreational use are different. METHODS: Identification strategies use a base line Difference in Difference, and a staggered Difference in Difference which estimates with relaxed assumptions and with a time varying treatment. RESULTS: Proportion of people who responded to use marijuana past 30 days increased by 1.82 percentage point from 4% baseline in MML states and 1.24 percentage point from the baseline 5% in RML states using linear regression after the legalization expansion. Base DID estimator did not find any statistically significant increase of marijuana use in MML states and only 0.72% increase in RML states. Days of using marijuana past 30 days increase slightly in both MML and RML states using linear regression. DID estimator was negative in RML states, making 16 days in RML states from the baseline 16.7 days. Staggered DID estimators for both marijuana use and days of marijuana use were negative. DISCUSSION: Compared to estimation from existing literature, DID estimators and staggered DID estimators have smaller magnitude of marijuana use and days of marijuana due to legalization expansion.