The proportion of cancer related mortality is 26.3% in 2004 and it is still increasing. The most common cancers are stomach cancer, lung cancer, liver cancer, colon cancer, breast cancer, and cervix cancer in order of frequency. The proportion of above 6 most common cancers in mortality is 66.8%. The purpose of cancer screening is to reduce morbidity and mortality of cancers by early diagnosis and early treatment. The assessment of screening test includes validity and reliability. Four terms describe the validity of a screening tests: sensitivity, specificity, positive predictive values and negative predictive value. The low prevalence lower the positive predictive value. Bias in the evaluation of screening tests lies in selection bias, lead-time bias, and length bias. National cancer screening program for 5 popular cnacers except lung cancer developed since 1999. The target population of this program is lower half of low-income level.