As company owners and top executives are still confused about what ISO 9001 may offer, this research study aimed to investigate if ISO 9001 effectiveness measured via the QMS rationale such as customer satisfaction focus, prevention of non-conformance and continuous improvement has significant effect on firms' performance through the determinants product quality, operational performance and business performance. The study utilized descriptive research design and convenience sampling technique covering 96 respondents from certified manufacturing firms in Laguna and Batangas. Responses were tallied and analyzed through linear regression analysis using SPSS version 20. Findings showed significant effect of ISO 9001 effectiveness on product quality, operational performance and business performance.The most essential findings noted in this research is the significant effect of ISO 9001 effectiveness and product quality on business performance contradicting previous empirical studies. An integral addition to the body of knowledge that addressed the controversial economic effect of ISO 9001. Generally, this paper depicted the significant effect of ISO 9001 on firms' performance when used effectively and optimally. As a matter of logic, an organization who struggles to demonstrate minimum compliance to QMS may not attain business excellence because ISO 9001 certification is not a guaranteed means for organizational survival. It is the firms' implementation effectiveness through objectives achievement that makes quality their best business plan.