Background and PurposePlatinum is the recommended adjuvant therapy for patients with resectable, anaplastic lymphoma kinase (ALK) -positive non-small cell lung cancer (NSCLC). Alectinib is an oral ALK tyrosine kinase inhibitor (TKI) that has shown significant efficacy in phase III clinical trials in patients with advanced ALK-positive NSCLC. For adjuvant therapy in patients with resectable ALK-positive NSCLC, the efficacy and safety of alectinib compared with chemotherapy are unclear. Methods In a global, phase III, open-label, randomized trial, patients with ALK-positive stage IB (tumor ≥4 cm), stage II, or stage IIIA NSCLC after complete resection (according to AJCC/UICC version 7) were randomized to receive alectinib (600 mg each, twice a day for 24 months) and intravenous platinum-based chemotherapy (4 cycles of 21 days each), respectively. The primary endpoint was disease-free survival (DFS), which was stratified in stage II or stage IIIA patients, followed by an intention-to-treat population. Other endpoints included central nervous system (CNS) -DFS, overall survival (OS), and safety. Results 257 patients were randomly divided into two groups, of which 130 received alectinib and 127 received chemotherapy. In stage II or IIIA patients, the 2-year disease-free survival rate was 93.8% in the alectinib group and 63.0% in the chemotherapy group (hazard ratio for disease recurrence or death, 0.24; 95% CI: 0.13-0.45; P<0.001); In the intention-to-treat population, 2-year disease-free survival was 93.6% in the alectinib group and 63.7% in the chemotherapy group (hazard ratio, 0.24; 95% CI: 0.13 to 0.43; P<0.001)。 Compared with chemotherapy, alectinib showed a clinically meaningful benefit in CNS-DFS (hazard ratio for CNS disease recurrence or death, 0.22; 95% CI: 0.08-0.58). OS has not been obtained and no unexpected safety events have been observed. Conclusion Alectinib significantly improves DFS compared with platinum-based chemotherapy in adjuvant therapy in patients with resected stage IB, II or IIIA ALK-positive NSCLC.