The article analyzes the dynamics of intellectual activity index (hereinafter -IAI) of the national innovation system (hereinafter -NIS) as an object of socio-economic development of the Republic of Belarus. Improving the methodology for strategizing the NIS development by introducing the methodology for IAI determining into the statistical database will help define the additional guidelines for its development. A comprehensive assessment of the NIS is performed by determining the IAI of its effectiveness. The developed structure of the IAI includes subindices: 1) "Management" to evaluate the institutional environment; 2) "Science" as a result of research and development; 3) "Intellectual resources" to determine the quality and number of researchers in the innovative production system; 4) "Innovations" to assess the efficiency of innovation. The article presents the structure of sub-indices from the point of view of organizing activities and evaluating efficiency, a set of indicators and their sources, and a calculation method. The utmost importance of intellectual resources determining the effectiveness of the NIS is emphasized. The authors offer the original definition of intellectual resources as a set of forms (physical, mental, social, emotional, spiritual, and adaptive), their use in the NIS determines their development and the increment of the latest knowledge. In the process of innovative activities, intellectual resources through the commercialization of scientific research, development of an intellectual product and the production of innovations determine the level of development of the intellectual capital of the national economic system. The article gives an example of calculating the IAI and its sub-indices for the NIS of the Republic of Belarus. The authors analyze the IAI dynamics and its sub-indices; they show the deviations of the elements that make up the sub-indices; the article identifies the development zones of the NIS of the Republic of Belarus. The growth of NIS IAI is inferred. However, negative trends in the sub-indices "Science" and "Intellectual Resources" cause the slowdown in the growth of IAI. Exposing the problem areas of the NIS of the Republic of Belarus, the article also offers guidelines for the improvement of the NIS of the Republic of Belarus.