Our study aimed to investigate the effect of Pap smear screening on stage at diagnosis of cervical cancer in a heterogeneous population of Thai women. Data was merged from the population-based cancer registry and screening registry based on unique identification numbers from 2006 to 2014. Patients being screened had lower odds to be diagnosed at late stage. After adjustment, married women had reduced risk of late stage cancer compared to single women. Muslim women had almost twice the risk of being diagnosed late stage compared to Buddhist women. The odds of being diagnosed at late stage decreased with increased number of screening. The probability of being diagnosed at late stage increased rapidly among females aged 40 to 55 years. Pap smear screening is a protective factor in diagnosis of late stage cervical cancer. Patients were more likely to be diagnosed at early stage with more frequent screening. For future screening programs, it will be beneficial to shorten screening intervals and take more concern for vulnerable population: women aged between 40 and 55 years, and women who are single or Muslim.