A mounting structure is provided for an NOx reduction system for a hydraulic excavator. The NOx reduction system includes a PM collection filter to be arranged in an exhaust passage of an engine to collect suspended particulate matter in exhaust gas from the engine, a urea SCR catalyst for reducing and purifying nitrogen oxides in the exhaust gas from the engine, a urea water spay device for spraying a urea water into the exhaust passage of the engine, a urea water feeder for feeding the urea water to the urea water spray device, and a urea water storage tank for storing therein the urea water to be guided to the urea water feeder. In the mounting structure, the counterweight is provided with a recessed section formed therein, the urea SCR catalyst is arranged in the recessed section, and the PM collection filter is arranged in the engine compartment. The urea SCR catalyst and PM collection filter are arranged to form an “L” pattern.