Research on monoclinic (M1) phase of VO2 has attracted a great of interest for smart coating applications due to its exceptional thermochromic property. Herein, we report the results using a novel approach to synthesize N-doped VO2(M1) thin films with high purity by heat treatment in NH3 atmosphere. The N dopant in the film can be regulated by varying NH3 concentration during the annealing process. We find that the N atoms are located at the interstitial sites or substitute oxygen atoms, and the V-N bonds in the VO2 thin films increase with NH3 concentration. The metal to insulator transition (MIT) temperature (tau(c,h)) of the VO2 thin film is effectively reduced from 80.0 to 62.9 degrees C, while the solar modulation efficiency (Delta T-sol) and the modulation efficiency at 2000 nm (Delta T-2000nm) are 7.36% and 55.6% respectively. The band gap of N-doped VO2 thin films related to MIT (E-g1) is estimated to be as low as 0.18-0.25 eV whereas the band gap associated with the visible transparency (E-g2) is about 1.50-1.58 eV. Based on the highly accurate first-principles calculations, the E-g1 of VO2 (M1) is reduced after substituted or interstitial N-doping, while the E-g2 alters with the mode of N-doping, which is excellent agreement with experimental measurement. (C) 2017 Elsevier B.V. All rights reserved.