Abstract: Aiming at the impact of the uncertainly cost estimates,we presented using fuzzy theory to estimate software development cost. By describing the principle of estimate formula of fuzzy theory and Mathematical Derivation, we created a mathematics model using fuzzy theory to estimate software development cost, gave the method of determining the membership function and computing similarity measure, discussed the relationship of database scale and software scale,analyzed the impact factor of database scale, designed the mathematics model of database scale estimation, pointed out the step of software development cost estimates using fuzzy theory. The summary of fuzzy theory method using fuzzy theory method to estimate the software cost can reduce the time cost of estimates, the complex of processing and the inaccurate of estimates which are caused by the variety of uncertain factors.

Key words: Fuzzy theory,Software development cose estimates,Membership function,Database scale estimation