Research had stated that there is increase in the number of people dying of blood diseases likewise there is a large number of people suffering from different kinds of blood diseases due to unavailability of human experts and inaccessibility of Haematology consultation. Hence this paper designs an expert system application for diagnosing selected blood diseases using rule-based method. The computer programming language employed was the C# programming language and MySQL as the Relational Database Management System (RDBMS). The results obtained showed that the expert system was able to successfully diagnose blood diseases corresponding to the selected symptoms entered as query.