Nitrate aerosol has become an increasingly important component of fine particles. While the formation and evolution of nitrate in airborne particles are extensively investigated, little is known about the formation of nitrate in clouds. Here we present a detailed investigation on the in-cloud formation of nitrate based on the size-resolved mixing state of nitrate in the individual cloud residual and cloud-free particles by single particle mass spectrometry, and the mass concentrations of nitrate in the cloud water and PM_2.5 at a mountain site (1690 m a.s.l.) in southern China. The results show a significant enhancement of nitrate mass fraction in cloud water and relative intensity of nitrate in the mass spectra of the cloud residual particles, underlining a critical role of in-cloud processing in the formation of nitrate. Based on the size distribution of relative intensity of nitrate in individual particles, we exclude the gas phase scavenging of HNO₃ and the facilitated activation of nitrate-containing particles as the major contribution for the enhanced nitrate. Regression analysis and theoretical calculations further reveal that nitrate is highly related (R² = ~0.6) to the variation of [NO_x][O₃], temperature and droplet surface area in clouds. Accounting for droplet surface area greatly enhances the predictability of the observed nitrate compared with using [NO_x][O₃] and temperature. Our results indicate a critical role of in-cloud formation of nitrate via N₂O₅ hydrolysis, even during the daytime, attributed to the diminished light in clouds. The detailed observation would benefit future investigations of the evolution and oxidative impacts of nitrate.