This study investigates the impact of ISO certifications on the Omani industrial and services companies listed on Mascut Securities Market. The firm performance measured by financial performance and market performance. The first one is measured by return on assets, return on equity and net operating profit while the market performance is measured by market fair value of shares. ISO certifications were surveyed if the company does or doesn't have the certificate. ISO certification effect on performance was investigated in 80 Omani firms from both industrial (60%) and services (40%) sectors. In the industrial sector, MANOVA test showed that the ISO has a significant effect only on NOP. In the services sector, MANOVA test showed that the ISO has a significant effect on market fair value and return on assets.