[en] A highly efficient transparent heater was intensively studied by optimizing a solution-processed indium tin oxide nanoparticles (ITO-NPs) film with oxygen vacancy control. To control the oxygen vacancy, ITO-NPs layers coated onto glass substrates was annealed under various atmospheric conditions such as vacuum, air, and argon (Ar). In result, all the solution-processed ITO films achieved high transparency of more than 96% in the visible range (500–730 nm) with respect to the glass substrate. As well, sheet resistance (R_s) decreased with increasing temperature and the lowest R_s (309.4 Ω/□) was achieved by annealing at 600 °C under Ar atmosphere. X-ray photoelectron spectroscopy (XPS) analysis revealed that the highest concentration of oxygen vacancies occurred under Ar atmosphere resulting in the highest carrier mobility and lowest R_s. Furthermore, application of the ITO-NPs films to transparent heater was attempted. In result, when applying 15 V, ITO-NPs film annealed under Ar atmosphere, showing the lowest R_s, exhibited the highest temperature (215.1 °C) among the samples. Also, highly thermal efficiency of 162.9 °C/(W/cm²) were achieved from the optimized sample. Thus, we suggest that the optimized ITO-NPs film was applicable to highly efficient transparent heater. - Highlights: • Highly efficient transparent heater was made with solution-processed ITO-NPs film. • Oxygen vacancy of ITO film was optimized with heat-treatment under Ar atmosphere. • The lowest R_s(309.4 Ω/□) and highest T₅₅₀(96%) were achieved with Ar atmosphere. • High heat-generating temperature (215.1 °C) was achieved with Ar atmosphere. • Highly thermal efficiency (162.9 °C/(W/cm²)) was achieved with Ar atmosphere.