Abstract: Nitrate easily accumulates in vegetables (especially leaf vegetables) and poses serious health hazard if excessively ingested. Light is a critical environmental factor influencing plant nitrate content. In this study, changes in contents of nitrate, soluble sugar and vitamin C (Vc) in hydroponic lettuce under 72 h continuous light and their correlation were analyzed. The aim of the study was to explore quality control measures of vegetables grown in controlled environments. The study showed that nitrate content in both leaf blade and petiole decreased significantly with variational rate in different time. Nitrate content in blade and petiole tended to steady after 24 h and 48 h continuous light respectively, while soluble sugar and Vc contents increased rapidly at a roughly constant rate and showed little trend of slowing down. There was no significant difference between blade and petiole soluble sugar contents. However, blade Vc content was much higher than that in petiole. Both in blade and petiole, nitrate content was significantly negatively correlated with soluble sugar and Vc contents. The results showed that short-term exposing to continuous illumination was suited to produce high quality lettuce with perfect unity of low nitrate accumulation and high soluble sugar and Vc contents.