A Dissertation Submitted in Partial Fulfillment of the requirement for the Degree of Master of Science in Procurement and Supply Chain Management (MSc-PSCM) of Mzumbe University – Dar es Salaam Campus.The world economy has been radically changed due to various reasons. The environment of business is characterized by rising complexity, uncertainty, instability and volatility. Companies have to re-think on their traditional methods and strategies of doing business to the pressure of changing market conditions, increased global competition, essential change in technology and shorter product life cycle. Managers are now realizing that no matter how strong and resourceful their firms might be; they are no longer able to maintain a competitive advantage. The problem is which factors are to be followed to select the supplier in private companies in Tanzania. The research based on the case study design. This enabled the researcher closely to observe the information and acquire detailed data. The study included staffs from three companies; purchase and Sales department, administration department, operation and production in Image Masters LTD, Procurement department in SHOPRITE and from Procurement department and Customer Marketing department at Serengeti Breweries LTD. Researcher used sample that comprise 20 respondents. The researcher used both Primary and Secondary data collection methods in which qualitative and quantitative techniques were used. The researcher uses factor analysis method to analyze data, which have been done through the assistance of Computer Program (SPSS Package). Manufacturing and service industry is moving towards higher value added activities thus; the requirements for suppliers might undergo significant changes. A questionnaire was sent to 20 employees in three companies, in order to identify the factor influencing supplier selection in private companies. Ten major categories of supplier selection criteria were identified prior to the survey. Results showed that the preferences are still focus on price/cost, delivery and quality, as reported in various related articles in the literature. Furthermore, the authors categorized the ten major criteria into qualifying criteria, selection criteria and addition factors that companies uses through the selection process. The findings can help the private and public supplier in selecting their suppliers.