This study aimed to conceptually introduce a novel method of dual-mode operation of nontoxic hypergolic thruster using decomposition gas of hydrogen peroxide (H2O2) and to experimentally assess the technical feasibility of the concept. An engineering model of a 500-N-scale nontoxic hypergolic thruster was tested with a nontoxic hypergolic combination that consisted of hydrocarbon fuel ("stock 3") and 95 wt % H2O2 as an oxidizer. Technically, dual-mode operation involves transition from a bipropellant mode to a monopropellant mode, or vice versa, which can provide great flexibility in terms of the deep throttling of the thrust level and the suppression of chugging instability at low thrust levels. The phase change of the oxidizer played a key role in the dual-mode operation. In particular, through hot-firing tests, system instabilities related to the mode transition were experimentally investigated. It was also confirmed that deep throttling of the thrust level was feasible through the mode transition.