Spherical SnO2:Sm3+ phosphors were successfully synthesized via a microwave-assisted solvothermal route. The developed microwave solvothermal process greatly reduced the required reaction time, and controlled the particle size and morphology of SnO2:Sm3+ phosphors. SnO2:Sm3+ phosphors can be excited at 318 nm, and emit a red light at 623 nm. When the microwave power was increased, the urea hydrolysis rate was accelerated, and the number of nuclei increases, thereby causing the particle size to reduce. The luminescence properties of prepared phosphors can be tailored via adjusting the irradiation power in the microwave solvothermal process.