Background: Over the past few years immunotherapy was incorporated in the first line treatment for advanced lung cancer. Obesity leads to a state of low grade chronic inflammation and plays a role in many pathophysiological processes including tumorigenesis. Recently evidence indicates that obesity and related inflammatory state may also impact the efficacy of immunotherapy. Methods: We conducted a retrospective study to look at the impact of body mass index (BMI) on the outcomes in patients with advanced lung cancer treated with immune check point inhibitors (ICI). According to their baseline BMI, patients were grouped into obese (BMI ≥ 30, n = 54) and non-obese (BMI < 30, n = 158). In another comparison using 25 as BMI cut off, patients were grouped into higher BMI (BMI ≥ 25, n = 105) and lower BMI (BMI < 25, n = 107). Results: From June 2012 to December 2020, 212 patients from our institute were included in this study. The median age was 64.5, average BMI was 26.4 kg/m², and 52% male sex. Median follow-up time in this study was 21.7 months. Patients with higher BMI using 25 as cut off had longer progression free survival (PFS) (8.1 months vs 5.3 months, p = 0.0034). Overall survival (OS) was better in higher BMI group than lower BMI group (10.8 months vs 5.9 months, p = 0.0008). The differences of PFS and OS for obese and non-obese groups were not statistically different (11.1 months vs 5.9 months, p = 0.157 for PFS; 12.0 months vs 6.9 months, p = 0.1512 for OS.). Conclusions: Our results suggest that excess weight may be associated with favorable PFS and OS in advanced lung cancer patients treated with ICI. Future studies are needed to investigate the underlying mechanism of this association.