This research is a descriptive study that aims to identify student’s mathematical thinking in aspect reasoning on grade XI in topic sequences and series through the inquiry learning model. The test is consists of one problem. The subject for this research were 3 students of XI IPA 1 class at SMA Negeri 1 Indralaya Selatan. The learning process adjusted with the inquiry phases. The data collection techniques used in this research are written tests and interviews. The result of this study indicates that subject 1 (S1) appears inductive reasoning only on the perception of generality and expression of generality in which logical analysis is gathering facts, assessing or proposing assumptions, and setting conclusions. Subject 2 (S2) appears all indicators of inductive reasoning and indicators of logical analysis, and subject 3 (S3) appears all indicators of inductive reasoning while indicators of logical analysis are submitting assumptions, assessing or testing assumptions, and setting conclusions.