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This study aims to examine the several factors influencing the efficiency of the hybridization of steel and polyethylene (PE) fibers in improving the compressive strength and tensile performance of ultra-high-performance fiber-reinforced cementitious composites (UHPFRCC). For the mechanical tests, three types of steel fibers (i.e., short straight steel (SS), medium-length straight steel (MS), and twisted steel (T) fibers) and four lengths of polyethylene (PE) fibers (i.e., 12 mm (SPE), 18 mm (MPE), 27 mm (LPE), and 36 mm (LLPE)) were hybridized. Each specimen included 2 vol.% of single or hybrid fibers, and the hybrid ratio was controlled by replacing 0.5% of the steel fibers with the same amount of PE fibers from 0 to 2%. Thus, a total of 7 single and 36 hybrid UHPFRCC specimens were fabricated. From the test results, it was found that the compressive strength decreased proportionally to the PE fiber content, but the decrease was more severe in hybrid specimens, including 1.5% PE fibers, than single fiber specimens, including 2.0% PE fibers. The tensile strength also decreased with an increase of PE fiber content, whereas strain capacity and energy absorption capacity per unit volume substantially improved with the inclusion of PE fibers. The SPE fibers showed the best hybridizing efficiency among PE fibers in improving the tensile strain capacity and energy absorption capacity of UHPFRCC, and the use of T fibers was the most effective in terms of cracking behavior. Keywords: UHPFRCC, Steel fiber, Polyethylene fiber, Fiber hybridization, Mechanical performance, Synergy effect