Firms‟ ownership may be used to increase firms‟ value and also solve problems associated with managers and shareholders. Firm size is gaining importance among researchers as far as performance is concern. To-date the government has divested several SOEs using different approaches among the most popular being; public offering, pre-emptive rights and competitive bidding and direct sales. The study intended to analyze relationship of firm ownership structure and size and performance listed state corporations in Nairobi Securities Exchange. Particularly, the study analyzed effect of government, local, foreign ownership and firm size   on financial performance of privatized state owned enterprise in Nairobi Securities Exchange. The study adopted a quantitative longitudinal research design to analyze this relationship and guided by relevant theories such as Growth of Firms Theory, Economic Theory, Stakeholders Theory and Dynamic Trade-Off Theory. The study targeted the 11 privatized listed firms in Nairobi Stock Exchange. The listed privatized State owned Enterprises were few but the period of the study was widened in order to give more accurate information. Data from secondary sources and mainly audited financial reports covering a period of 10 years (2008-2017) were analyzed for the purpose of the study. The study used mean, standard deviation to describe the data and inferential statistics used to describe the relationship between ownership structure and size on financial performance and tested using Panel Regression analysis. The study was informative to Capital Market Authority, Central Bank and NSE policy regulators in providing insight from the study for designing policies to that will improve privatization process, regulation on firms‟ ownership and size to control any dominance. Government shares was the only variable found to have a significant effect on ROA with a p value of 0.000 holding local shares, foreign shares and firm size constant which all had insignificant effect. Relationship between ownership structure and firm size on ROA remained statistically significant when interest rate was introduced as moderating factor with a p value of 0.0005 whereas for ROE the relationship was statistically insignificant when interest rate was introduced as moderating factor.