Understanding the nature of science (NOS) is an important goal of science education, and textbooks are a key factor in shaping students' conceptions of NOS. In this study, we analysed NOS represented in middle school chemistry textbooks in the Chinese mainland. The selected materials were three most commonly adopted textbook series, which were published in 2001, 2001, and 2012. The analysis was conducted from 11 aspects -- empirical, inferential, creative, theory-driven, tentative, myth of 'the scientific method', scientific theories, scientific laws, social dimensions of science, social and cultural embeddedness of science, limitations of science -- to determine the distribution of different representation types (explicit-informed, implicit-informed, implicit-naïve, and explicit-naïve) in each NOS aspect in each textbook series. Textbook analysis results indicated that each textbook series had mixed conceptions of NOS and most NOS representations were informed and implicit. The frequencies of NOS representations in the selected textbook series were roughly identical; however, the last two textbook series had relatively higher error rates than the first one. Moreover, there was an overall imbalance in the representations of NOS aspects. Our findings, combined with previous studies, were also discussed.