Countries and societies in general facing a multitude of social problems emanating from various sources such as environmental degradations, hunger and diseases.  Among such problems commonly listed are AIDs, overpopulation, drug abuse, and mistreatment of females, use of tobacco product, and behaviour that increases the chances of heart disease. To deal with these challenges, multi sectoral approach is required where all stakeholders including, non-governmental organization, community based organizations, government, religious institutions and voluntary associations play their roles.  This study investigated effect of strategic social marketing on effectiveness of Community Based HIV and AIDs organizations (CBOs) in Nairobi County, Kenya.  The study was guided by positivistic research philosophy.  A cross-sectional survey research design was employed.  The population of the CBOs comprised 350 organizations and area sampling technique was used.   A sample of 183 CBOs was used in this study from eight constituencies which were proportionately represented.  Random sampling was used to select sample elements.  Descriptive statistics were used to profile characteristics of the surveyed organizations and the respondents. Correlation and multiple regression analyses were used to evaluate the relationships between strategic social marketing and effectiveness as well as to assess its effect.   The findings indicate that strategic social marketing has an influence on the effectiveness of Community Based HIV and AIDs Organizations in Nairobi County.  The study concluded that for CBO to design appropriate programmes, define realistic output and outcome levels, they should employ strategic social marketing approach as this influences their effectiveness leading to better performance.  This study recommends that managers and practitioners should participate especially in the National HIV and AIDS policy development in order to inform their own policies and activities, as this not only improves their ability to identify the expected outcomes, but also enhances their effectiveness. 

KEY WORDS: Strategic social marketing, Efficiency, Community-based organizations