The purpose of this study is to explore inter-grade dividing criteria of the 2015 grade group elementary English textbooks. Elementary English textbooks consist of two grade groups: the 3rd and 4th grade group and the 5th and 6th grade group. L2 Syntactic Complexity Analyzer(L2SCA) is utilized to investigate the dividing criteria of the communicative functions implemented in these textbooks. Subjects of the analysis were the listening dialogues for their structural sequencing of 3rd to 4th graders and 5th to 6th graders separately within their own grade groups. Data were processed and analyzed using SPSS 25.0 and independent sample t tests for inter-grade textbooks for 14 L2SCA statistical indices. The findings are: 6 indices out of the 14 L2SCA statistical indices turn out to be critical for dividing the 2015 grade group elementary English textbooks. The 6 indices are mean clause length, mean sentence length, mean t-unit length, mean complex noun phrasal number per phrase and per t-unit and mean number of verb phrases per t-unit. Based on the findings, it is suggested that a standardized criteria derived from these 6 indices can be provided as an inter-grade dividing criteria of the 2015 grade group elementary English textbooks.