By the PLIF optical technique, this study focused on the development of the impinging type injector design of the 100-lbf level monomethylhydrazine/nitrogen tetroxide liquid rocket. The design of the multi-pair impinging injector plate is based on the cold flow and hot fire observations of unlike-doublet impinging jets of monomethylhydrazine/nitrogen tetroxide and their simulants. The mass distributions of the impinging jets from the cold flow observations are used to arrange the paired orifices of the impinging unit on the injector plate through a image projection method. For the most uniform distribution and the best overall estimated C* of the overall spray, the orifice size of the impinging element and the configuration of the paired orifices of an injector plate was determined for the prototype injector plate design. Static tests of the ground-test liquid rocket engine with the designed injector plate are performed. The measured sea-level thrust of the engine was 62.1 lb, and the characteristic exhaust velocity reached 1429.2 m/s at a total propellant mass flow rate of 153.4 g/s. The estimated vacuum thrust was 99.8 lb, and the vacuum Isp was 295.8 s as well. The combustor pressure of the liquid rocket engine was 153.2 psi and remained stable during operation. During the 17 seconds of the hot fire test, the highest wall temperature was measured at the nozzle throat, which was about 1000K.