Abstract: Polypropylene (PP) insulation material is recognized as the next generation of cable insulation material due to the excellent electrical properties and recyclability. In order to further enhance the performance and service life, feasibility of reducing insulation thickness of PP cables relative to cross-linked polyethylene (XLPE) cables was analyzed. Withstand field strength of PP insulation material under different voltages was determined at operation temperature through step-by-step breakdown test and life index test. By the tests mentioned above, optimization for insulation thickness of medium voltage PP cables was carried out. Insulation thickness of 8.5 mm and 3.5 mm were proposed for 26/35 kV and 8.7/10 kV PP cables respectively, and type tests on PP cables with optimized insulation thickness were conducted. Results showed that reliability of cables with optimized insulation thickness met the requirements. Besides, current carrying capacity of PP cables could be increased after thickness reduction.