The paper deals with developing he mathematical model of nonorthogonal inertial measuring instrument based on triaxial MEMS gyroscopes. Both the model of the sensitive element and realization of the algorithm of transforming measurement information are represented. The model of the sensitive element takes into consideration measuring errors of typical MEMS gyroscope. The model is based on characteristics represented of technical description given by manufactures (Analog Device). The algorithm of information processing, which includes the restoration matrix, is developed. The table of direction guides is obtained. The effectiveness of the developed mathematical model is proved by using Simulink model, which takes into consideration elements inherent to real inertial measuring instruments. The results of researching accuracy and the possibility of increasing the dynamic range are represented.