9The present study explored the changes in grit’s two subfactors(perseverance of effort and consistency of interest), behavioral regulation, and self-determination for middle school students, and examined their longitudinal causal relations. To do this, we administered surveys to 436 second graders in four middle schools three times over beginning of second grade(t1), end of second grade(t2), and end of third-grade first semester(t3), and conducted the autoregressive cross-lagged model analysis. Main results are as follows. First, scores at previous timepoints had stable predictions of scores at following timepoints. In particular, lower autoregressive coefficients for grit than for self-determination suggested that the changeability of grit would be more dynamic than that of self-determination. Second, the longitudinal reciprocal causal relations between perseverance of effort and self-determination and between consistency of interest and behavioral regulation confirmed that they grow together through reciprocal positive effects. This result implied that consistency of interests would have indirect effects on academic achievement-related variables by showing that consistency of interests at t1 had positive effects on self-determination at t3 through perseverance of effort at t2. In addition, the result, which showed that consistency of interest at previous timepoints had positive effects on perseverance of effort at following timepoints, implied that perseverance could be strengthened by maintaining consistent interests. This short-term longitudinal study confirmed the possibility of change in grit and suggested that positive academic motivation can be formed through the change in grit.