Background: Acquired EGFR mutations (C797, L792, G796) co-occurring with T790M were reported to lead the resistance to osimertinib. It was reported that for advanced NSCLC patients, exon 19 deletion (19del) might be associated with longer PFS compared to L858R mutation accepted EGFR-TKIs therapy. In this study, we try to analyze the difference of the resistant mutation spectrum in patients accepted osimertinib treatment carrying these two types of mutations. Methods: Using targeted gene capture and next-generation sequencing technologies, we analyzed the somatic mutations in 110 NSCLC patients (pts) that were clinically resistant to osimertinib. Results: All of the 110 patients had an EGFR activating mutation (19del in 64 pts and L858R in 46 pts) and 104 were EGFR T790M positive. EGFR mutations which may lead to the acquired resistance of osimertinib were identified in 52.7% (58/110) patients, including C797S (43 pts), C797G (6 pts), L792H (6 pts), L792V (2 pts), G796S (3 pts), G796C (1 pts), L718V (4 pts), L718Q (7 pts), and 18 patients had multi-clonal resistant mutations. In all analyzable patients, at least one resistant mutation was identified in cis with T790M, and two of them also carrying an in trans mutation who were also co-existent with L858R. No patient had only in trans mutation in our study. The resistant mutation were more frequently detected in 19del group than in L858R (62.5% vs 39.1%, p= 0.015), with 21.9% vs 10.9% patients had mult-clones and 15.6% vs 8.7% EGFR amplification. These may due to the larger drug selection pressure from longer treatment of 19del group. Bypass or downstream co-occurring activating mutations detected were EML4-ALK rearrangement (1 pts), MET amplification (1 pts), RET-CCDC6 rearrangement (1 pts), KRAS mutations (1 pts), BRAF mutations (7 pts), PIK3CA mutations (5 pts), and PTEN deficiency (3 pts). Conclusions: In our study, acquired EGFR mutations leading to osimertinib resistance were more likely to be identified with EGFR 19del than L858R mutation NSCLC patients. Understanding these resistance mechanisms may be useful to develop more effective therapies for patients resistant to osimertinib.