Aims: To investigate the effectiveness of LDV/SOF and the impact of RAS on the treatment outcome in Mongolian CHC patients. Introduction: Mongolia has the highest prevalence of hepatitis C virus (HCV) infection worldwide. Ledipasvir/sofosbuvir (LDV/SOF) was introduced to Mongolia since 2016 for HCV eradication. It has been reported that HCV resistance-associated substitutions (RASs) would affect theeffectiveness of LDV/SOF in western chronic hepatitis C (CHC) patients. Methods: Patients with genotype (GT) 1b HCV infection were prospectively enrolled inMongolia and treated with LDV/SOF for 12 weeks. The proportion of pre-treatment NS5AY93H RAS in viral quasispecies was measured with next-generation sequencing. The endpointof LDV/SOF effectiveness was sustained virological response at post-treatment week 12(SVR12). Results: A total of 94 CHC patients were evaluated. The baseline Y93H proportion was <1% in74 patients, 1e15% in 7, 15e50% in 2, and 50% in 11. All patients completed 12-week LDV/SOFtreatment and the SVR rate was 90.4%. The rate of failure to achieve SVR12 for patients withY93H < 1%, 1e15%, and _15% were 0%, 14.3%, and 61.5%, respectively (p for trend Z0.001). Inunivariable analysis, older age, baseline alanine transaminase level <40 U/mL, and a higherproportion of Y93H were associated with treatment failure. In multivariable analysis, only a higher proportion of Y93H was associated with treatment failure (p Z 0.022). Conclusions: LDV/SOF therapy achieves a high SVR rate in Mongolian CHC GT1b patients withoutbaseline Y93H RAS. A higher proportion of Y93H may severely undermine the effectiveness ofLDV/SOF.