In estimating of software size, the FP (Function Point) method has many strengths in comparison with others, but it still has many controversies about the complexity or the weight value. In this paper, I investigated several cases where FP does not reflect complexity in various functions of a software, improving way of measuring UFP. To do so, new factor including interaction degree between functions and the weight values is introduced. Existing business software being used in our company is applied in order to analyze the cases and derive the improved UFP model. In addition, a new weight value resulted from interaction degree between functions is applied with various numerical experiments and analysis of correlation between each variables. For verification, proposed model has been applied to another business application, proving improved model of UFP with the same trend.