Cancer screening evaluation will be a important issue in Korea in near future. Bias in cancer screening evaluation is well known by concept, but it is not applied yet using the data from screening program of Korea. So introducing the way how the bias is adjusted will be helpful. This review deals with the type, meaning, the way how the bias is adjusted, and examples. Especially, lead-time bias, length bias and self-selection bias were focused. Adjusting bias is one of the imperative step for epidemiologic analysis. Understanding the background concept and experience using the screening program data will be helpful for estimating the effectiveness of national screening program of Korea.