Given the longitudinal nature of L2 grit, the use of conventional research methodologies with cross-sectional data to examine the validity of L2 grit scale seems inadequate. The present research was an attempt to extend the domain-specific phase of research on L2 grit, with the pursuit of long-term goals at its core, into a dynamic one. Thus, we adopted a longitudinal confirmatory factor analysis-curve of factors model (LCFA-CFM) approach to trace changes in L2 learners’ grit at different points of time in an EFL course. LCFA-CFM ensures measurement invariance over time, deals with second-order latent variables, takes into account measurement errors, and is capable of assessing interindividual differences. With this in mind, we, first, employed LCFA to test the factor invariance of L2 grit based on a bifactor CFA model over time and, second, used CFM to measure change of L2 grit during an L2 course. To do so, we collected data from 437 adult EFL learners in Iran in four time phases using the L2 grit scale and analyzed them using Mplus 7.4. The model fit was accepted and invariance of the latent factor of L2 grit was confirmed over time. Also, the negative covariance between initial level of L2 grit and its rate of change over time (second-order latent variables) suggested a steeper increase in the construct over time for learners with lower initial scores of the construct. That is, L2 learners who started at a higher level of L2 grit experienced less change in L2 grit over time. The LCFA-CFM ensured that the factor structure of L2 grit is invariant over time and provided insights into how L2 grit changes over an L2 course.