The development of non-precious metal based electrocatalysts for hydrogen evolution reaction(HER)holds a decisive key to a spectrum of energy conversion technologies.Previous studies have established layered molybdenum chalcogenides as promising candidates.1-2 In this work,we prepared ultrathin MoS2(1-x)Se2x alloy nanoflakes with monolayer or few-layer thickness and fully tunable chemical composition for maximal HER activity.Spectroscopic characterizations corroborate the progressive evolution of their structures and properties as x increases from 0 to 1 without any noticeable phase separation.In particular,it is evidenced that the introduction of selenium continuously modulates the d-band electronic structure of molybdenum,probably leading to tuned hydrogen adsorption free energy and consequently electrocatalytic activity.Electrochemical measurements show that all MoS2(1-x)Se2x nanoflakes are highly active and durable for HER with small overpotentials in the range of 80-100 mV and negligible activity loss up to 10,000 cycles.Most importantly,alloyed nanoflakes,especially with the chemical composition of MoSSe,exhibit improved performance compared to either MoS2 or MoSe2.Given their overall similar nanoflake morphologies,we believe such improvements reflect the higher intrinsic activity of alloyed catalysts with the hydrogen adsorption free energy closer to thermoneutral.