This study sought to assess the effects of competition on the performance of letter mail at the Postal Corporation of Kenya. The literature reviewed was based on three theories: porter's five forces theory, Game theory and Chamberlain's monopolistic theory. The study adopted a case study design. The target population of the study was Postal Corporation of Kenya employees at Nakuru, Cyber cafes along Kenyatta Avenue in Nakuru, G4S offices in Nakuru, Nation Group of Newspaper office Nakuru and Bus company offices at Nakuru bus terminus. The study found that competition has had far reaching negative effects on the performance of letter mail at Nakuru post office. The latest advancement in communication technology have created fierce competition to letter mail communication and the researcher has therefore recommended adaptation of latest technology at PCK Nakuru coupled with implementation of strategies such as mergers/acquisitions with strategic industry partners, automation of business processes and provision of quality service.