ISO 9000 registration has a worldwide acceptance as an important quality indicator. Since it was first published in 1987 there has been a considerable increase in the number of organizations that applied for and got the certification. However, some opposing ideas on the effectiveness of ISO 9000 registration began to arise in recent years. In this study the effects of ISO 9000 registration on the performance of companies are analysed from the two aspects: stock performance and financial performance. Two quantitative methods, event study and ratio analysis are used in these analyses. The results reveal that there are no significant positive effects of certification on the companies’ performance.