This study investigates the development of lexico-grammatical complexity in two levels of graded readers, beginner and intermediate. Using the linguistic features described in Biber et al. (2011), 58 graded readers were analyzed. Biber et al. (2011) proposed a lexico-grammatical developmental sequence based on empirical research. In this study, 11 of these grammatical features were counted in these two levels of graded readers. The graded reader corpus was tagged for the lexico-grammatical features, and a Mann-Whitney U test was conducted to account for the differences between graded readers' levels. The results of the statistical analysis suggest that beginner graded readers have more advanced features of complexity than intermediate graded readers. Nevertheless, after close analysis of text excerpts, it is clear that this is a result of the register being investigated, with intermediate graded readers having more features of conversation than beginner graded readers.