$<$h2$>$Abstract$<$/h2$><$h3$>$Background$<$/h3$><$p$>$The quality of primary care is central to China’s ongoing health system reforms. Although there is ample evidence on investments in health system infrastructure, little objective evidence is available on the quality of care delivered to patients, particularly by rural grassroots providers. The aim of this study was to assess the ability of rural primary care providers to diagnose and treat two common diseases.$<$/p$><$h3$>$Methods$<$/h3$><$p$>$We deployed unannounced standardised patients (SPs) in a representative survey of village and township level primary care providers located in three prefectures in Sichuan, Shaanxi, and Anhui provinces. One case presented symptoms of unstable angina and another described symptoms of his or her child with diarrhoea. Physician performance in interactions with SPs was assessed against international and national standards of care yielding quality measures for clinical process (adherence to checklists of recommended questions and exams) and treatment. We used regression analysis to assess correlates of quality. We obtained approvals from the institutional review boards of Stanford University, CA, USA (FWA00000935) and Sichuan University (K2015025), China. Informed consent was obtained verbally from all providers participating in the study. All individuals who participated as SPs were trained to protect themselves from any invasive tests or procedures.$<$/p$><$h3$>$Findings$<$/h3$><$p$>$We successfully completed 293 standardised patient interactions in July 2015. On average, interactions lasted 9$·$57 min. Providers completed 20% (95% CI 17– 23) of recommended checklist items in village clinics and 25% (23– 27) in township health-care centres. Only three (9%, 2– 24) of 33 of village doctors and 15 (14%, 8– 23) of 104 township doctors correctly treated the diarrhoea case according to national standards. In treating angina, 36 (67%, 53– 79) of 54 village doctors and 60 (59%, 49– 68) of 102 township doctors correctly referred patients. Doctor education, but not the value of facility infrastructure or equipment, was predictive of quality of care.$<$/p$><$h3$>$Interpretation$<$/h3$><$p$>$We found evidence of low-quality health-care provided by grassroots providers in China’s rural health system. Investments in facility infrastructure poorly proxy the quality of care. Policies to improve the quality of primary care among grassroots providers should be prioritised in current reforms.$<$/p$><$h3$>$Funding$<$/h3$><$p$>$111 Project (B16031), the World Bank’s Knowledge for Change Program (7172469), the Fundamental Research Funds for the Central Universities and the Research Funds of Renmin University of China (2015030245).$<$/p$>$