Paper is devoted to the problems of overhead power lines (OHL) state assessment. Special attention is given to its method and technics overview. Urgent OHL maintenance problems and possible reasons of breaking its operation were defined based on OHL failure statistical analysis. New diagnostic system for these problems solving is proposed. The system consists of two subsystems: unmanned aerial vehicle (multicopter) with modular diagnostic blocks and OHL state assessment system. OHL state assessment is performed by means of artificial intelligence techniques, which input is data obtained by multicopter. Also the paper contains description of diagnostic multicopter prototype. Performed analysis shows that proposed system allows to solve urgent problems of OHL maintenance and to improve its reliability.