This study investigated outpatient prescription medicines use in primary health care institutions to evaluate the impact of the essential medicines policy on rational medicines use. Data were collected from primary health care institutions in 4 province-level areas of different socioeconomic status in China. Around the policy implementation, the numbers of various medicines used per prescription were compared by t test and 1-way analysis of variance, and the proportions of prescriptions with antibiotics, injections, or hormones were compared by chi-square test. After the policy implementation, the numbers of all medicines and antibiotics used per prescription dropped, and the number of essential medicines used per prescription increased in all 4 areas. The percentage of prescriptions with injections declined in all 4 areas, but the percentages of prescriptions with antibiotics or hormones witnessed different status in different areas. To a certain degree, the essential medicines policy in China has taken effect on rational medicines use but needs development.