Abstarct: The Rule baxsed Expert Systems are used in many different aspects. In Fuzzy Expert Systems, rules are used for Fuzzy inference. In the case of suitable number of rules and Full cover different scenarios System, Fuzzy Expert Systems will be divided into certain and uncertain groups. A certain system, with enough necessary rules can support all the input data and activate at least a rule for each input. For uncertain systems, there is at least one input that will not trigger any rule to inference and present output. In this research, we study the shortage of rules. To do so, first the shortage of rules in the system will be diagnosed and then the missed rule will be injected into system. Using fuzzy rules interpolation to calculate the least square error method, we try our best rule established candidate to replace the missed rule. In the following the commentary method with concentration on the available Fuzzy Rules Interpolation will be compared.