Background: Depth of response (DpR; max% reduction from baseline in sum of target lesion diameters), has been reported to be associated with improved prognosis in several malignancies. However, there have been few reports regarding DpR in patients with locally advanced pancreatic cancer (LAPC). Thus, we investigated the association between DpR and overall survival (OS), and whether there were any differences in treatment efficacy on DpR between modified FOLFIRINOX (mFFX) versus gemcitabine plus nab-paclitaxel (GnP) as 1st-line chemotherapy for LAPC, using the results from JCOG1407 which was a randomized phase II trial comparing these regimens. Methods: Of the 126 patients enrolled in JCOG1407, patients were eligible except for the followings; patients who never had an imaging study or had only non-target lesions. The association between DpR and OS was investigated by dividing DpR into 3 groups at the tertile point (T1, T2 and T3 from largest to smallest), and whether DpR contributes to OS was examined by multivariable analysis. The differences in treatment efficacy between both regimens were investigated byevaluating DpR, time to DpR, and duration of response (DoR). Results: A total of 109 patients were eligible for this study (n = 53/56 in mFFX/GnP). The number of patients in T1, T2, and T3 were 37, 36, and 36, respectively. The median OS of T1, T2, and T3 were 29.3 (95% CI, 21.0-NE), 20.6 (95% CI, 15.8-24.5), and 19.0 months (95% CI, 12.7-22.4), respectively (P= 0.0237). Multivariable analysis identified DpR as an independent prognostic factor for OS (HR 1.883, 95% CI 1.030-3.442, P= 0.040 for T2 vs. T1, and HR 2.523, 95% CI 1.342-4.744, P= 0.004 for T3 vs. T1). The median DpR in GnP was better than that in mFFX (28.9 vs. 22.7%; P= 0.041). The median DoR in mFFX tended to be longer compared to that in GnP (8.2 vs. 5.3 months; P= 0.132). No differences in mean time to DpR were observed between the two regimens. Conclusions: In LAPC patients receiving 1st-line chemotherapy, larger DpR contributed to OS. Although OS and PFS between mFFX and GnP were similar, the treatment efficacy on DpR and DoR might be different between the two regimens. Clinical trial information: UMIN000023143.