The present work is an attempt to develop an expert system to diagnose hypomelanosis skin diseases with the ways of treatments. The required knowledge acquired from a domain expert (Dr.Rabab Noory Al-Saady) and medical textbooks. The system consist of five module and used a production rule knowledge representation system. The inference engine module use a backward chaining strategy while it uses a B+-tree and depth first search (DFS) as a search strategy. The system consider reasoning under uncertainty using certainty factor method. Further more, the system support two types of explination Why and How. "Why" concerned with why the system ask the specific question, while "How" is concerned with how the system arrive the specific conclusion. The system provides an interface for the users and human experts that enable the human experts to add, delete, update, view knowledge of a specific disease. The system has been implemented by using the visual prolog programming languge to built expert interface tool and built the inference engine.