Performance measurement in the service industry is a critical issue for identifying how well is the company working. In manufacturing companies, performance measurement primarily relies on capacity and the number of products produced without defects. How close is the final product to the targeted product is a tool for observing the effectiveness of production process. In the banking industry, the performance depends on revenue - cost analysis. Companies in service industry need to understand how successful their business goes; several techniques developed for measuring their performance. Most of those techniques need too many parameters to observe and prioritize for reflecting relative effects of individual criteria into the analysis. It is essential to specify each of them because more criteria mean more dimension and high accuracy. The criteria involved in law firms' performance measurement are uncertain, vague and collected at different periods. The purpose of this study is to observe longitudinal performance evaluations in law firms by using dynamic intuitionistic fuzzy sets.