Abstract： Background: IMpower 133 trial first confirmed the efficacy and safety of adding atezolizumab or placebo to first-line treatment with chemotherapy in patients with extensive-stage small-cell lung cancer(SCLC).While,overprice limited its broad use in clinical.The aim of this study was to evaluate the cost-effectiveness of atezolizumab plus chemotherapy in treatment of extensive SCLC as first line in China.Methods: A Markov model was established by extracting data from the IMpower 133 trial with untreated extensive SCLC patients.Utility values were obtained from published studies,and the costs were acquired from real world and literature.Additionally,sensitivity analyses based on a willingness-to-pay(WTP)threshold were performed to identify the uncertain parameters of Markov model.Results: Total costs of atezolizumab group were $48,129,while cost of chemotherapy alone was just $12,920 in placebo group.The quality-adjusted life-years(QALYs)in atezolizumab group was just 0.072 higher than that in placebo group(0.858 QALYs vs.0.786 QALYs).The cost-effectiveness ratio between atezolizumab combination with chemotherapy and chemotherapy alone was$489,013/QAL Y in China.The net benefit of placebo group was significantly higher than atezolizumab group.One-way sensitivity analyses highlighted that utilities of the progression-free survival(PFS)and progression disease state in placebo group were the most influential parameter.Conclusions: Atezolizumab combination therapy was not more cost-effective than chemotherapy alone at a WTP threshold of $25,929/QALY in China.