We have developed a hydroxylammonium nitrate (HAN)/hydrazinium nitrate (HN)-based low-toxicity monopropellant [high-performance, no-detonation propellant (HNP)] that has safety characteristics such as no autocatalytic reaction (no autocatalytic reaction: combustion cannot continue without a source of heat) and no detonation. However, its specific impulse (Isp), a rocket engine performance indicator, was lower than that of hydrazine. Therefore, we investigated many types of compositions and found methanol to be suitable as a fuel ingredient for increasing the Isp of the developed propellant and reducing its viscosity. We produced the developed monopropellant consisting of HAN/HN/methanol/water and having a low viscosity and an Isp of 260 s at the laboratory scale.