EGFR exon 20 insertion (20ins)-positive non-small-cell lung cancer (NSCLC) is an uncommon disease with limited therapeutic options and dismal prognosis. Here we report the activity, tolerability, potential mechanisms of response and resistance for dual targeting EGFR 20ins with JMT101 (anti-EGFR monoclonal antibody) plus osimertinib from preclinical models and an open label, multi-center phase 1b trial (NCT04448379). Primary endpoint of the trial is tolerability. Secondary endpoints include objective response rate, duration of response, disease control rate, progression free survival, overall survival, the pharmacokinetic profile of JMT101, occurrence of anti-drug antibodies and correlation between biomarkers and clinical outcomes. A total of 121 patients are enrolled to receive JMT101 plus osimertinib 160 mg. The most common adverse events are rash (76.9%) and diarrhea (63.6%). The confirmed objective response rate is 36.4%. Median progression-free survival is 8.2 months. Median duration of response is unreached. Subgroup analyses were performed by clinicopathological features and prior treatments. In patients with platinum-refractory diseases (n= 53), confirmed objective response rate is 34.0%, median progression-free survival is 9.2 months and median duration of response is 13.3 months. Responses are observed in distinct 20ins variants and intracranial lesions. Intracranial disease control rate is 87.5%. Confirmed intracranial objective response rate is 25%.