Addition of manganese in elements of matrix for internal-tin Nb3Sn strands has been investigated. Two types of multifilamentary strands with distributed source of tin, 1.8 wt.% Mn in elements of matrix before reaction heat treatment and 1 wt.% Mn in matrix after reaction, have been fabricated by internal-tin method. One of the produced strands had reinforced stabilizer from CuNb nanocomposite alloy. The increase of Nb3Sn layer growth rate in superconductors filaments of manganese doped strands was determined. Microstructures of Nb3Sn filaments after reaction heat treatment were investigated by SEM and TEM microscopes. The critical current density in magnetic fields up to 20 T, hysteresis losses, critical temperature and mechanical characteristics were measured. The increase of critical current density in internal-tin Nb3Sn strands with manganese doped matrix and the formation of fine microstructure Nb3Sn layers was revealed.