Software companies try to upgrade the level of technology and quality of their products by adopting Software Process Improvement (SPI). First of all, the companies must understand affecting factors based on various environmental view points in order to acquire competitive advantage. In this study, a research model gets started on the basis of literature reviews, and identifies SPI factors that affect the maturity levels of organizations. Organizational members and assessment types are chosen as moderating factors. An empirical research has been made for analyzing the causal relationships among affecting factors and CMM levels. The results of this study will suggest several implications to decision makers who handle SPI issues.