Chemoradiotherapy (CRT) followed by consolidation immune checkpoint inhibitors significantly improves survival in unresectable locally advanced non-small cell lung cancer. However, the optimal sequence for CRT and immune checkpoint inhibitors has not yet been established. We investigated the dynamics of peripheral blood immune cells during CRT to determine the best sequence for treatment. Peripheral blood samples were prospectively collected pretreatment, weekly during CRT for 6 weeks, and 1 month posttreatment in 24 patients with locally advanced non-small cell lung cancer who received definitive CRT. Immune cell analysis was performed by flow cytometry. Ex vivo PD-1 blockade assays were performed by IFN-γ intracellular cytokine staining. Lymphopenia was prominently observed during CRT and mostly recovered 1 month post-CRT. Robust proliferation of CD8 T cells was induced, peaking in the last week during CRT and decreasing post-CRT. The robust proliferation of CD8 T cells led to an increase in the frequency of CD28 CD57 replicative senescent and terminally differentiated cells post-CRT. Tumor-reactive CD8 T cells increased during CRT and peaked in the last week. One month post-CRT, the frequency of tumor-reactive CD8 T cells decreased and TOX TCF1 terminally exhausted CD8 T cells significantly increased. Anti-PD-1-induced functional restoration of PD-1 CD8 T cells was maximized in the last week of CRT and significantly decreased post-CRT. The findings suggest that earlier administration of PD-1 blockade may be associated with superior efficacy compared with delayed administration after completion of CRT. These findings provide an immunologic rationale for optimal timing of combining immune checkpoint inhibitors with CRT in clinical trials.