During the last decade, software industry has concerned more about the quality of software process, as indicated by increasing number of proposed software process management (SPI0 standards and models. However, the adoption rate of these standards and models in software development is still low and the failure rate of SPI implementations is very high. For these reasons, this paper is aimed at investigating the key success factors for SPI implementation to answer two research questions. The first question is, what factors do respondents think that they have an impact on SPI implementation success?; and the second question is, does the organizational size have moderating effect on the relationship between human factorsi organizational factors and SPI implementation success? The conceptual model and hypotheses of this study are proposed by extending and integrating conceptual models from prior research, collecting the human, organizational, and implementation factors associated with SPI implementation in industry. Furthermore, the contextual factor that is organizational size was added to the model to test the sensitivity of the variation in software organizations. Then, a quantitative questionnaire survey was gathered from 23 software organizations in Thailand which apply CMMI (Capability Maturity Model Integration) as their SPI guidelines in order to test the conceptual model and hypotheses of the study. The results indicate that SPI implementation success depends on six factors, i.e., management commitment, staff involvement, allocated resources, pilot projects setting SPI goals, and defined process model, while organizational size has no moderating effect on SPI implementation success. Finally, the main contribution of this paper is the guidance for SPI practitioners on key factors that practitioners need to focus during an SPI implementation. Moreover, this guidance will be useful in the process of allocating resources and prioritizing tasks to effectively implement SPI in the organization.