We report a new kagome quantum spin liquid candidate CuaZn(OH)6FBr,which does not experience any phase transition down to 50mK,more than three orders lower than the antiferromagnetic Curie-Weiss temperature (～200 K).A clear gap opening at low temperature is observed in the uniform spin susceptibility obtained from 19F nuclear magnetic resonance measurements.We observe the characteristic magnetic field dependence of the gap as expected for fractionalized spin-1/2 spinon excitations.Our experimental results provide firm evidence for spin fractionalization in a topologically ordered spin system,resembling charge fractionalization in the fractional quantum Hall state.