The role of information system(IS) audit is growing more important according to rapid change of Information technology. This article is to introduce what is IS audit, purpose and effect of IS audit. Now most clients of private IS audit services companies are the public sector. Korean public sectors invest a lot of money to build or implement their information system. Most of their systems are developed by system intergration companies. But they do not have professional engineers to evaluate and review outsourced information system. Therefore they must use outside private professional engineers for sysem auditing. We, including writer, established the first IS audit sevice company in Korea on September in 1997. After that about 15 IS audit service companies are established until now. The effect of IS audit is highly evaluated In public sector by the clients Most clients think IS audit service contributed to upgrade the quality of software and standard the methodology of developing system