We studied the effective exciton radiative lifetimes of monolayer two-dimensional transition-metal dichalcogenides MX2 (M = Mo, W; X = S, Se). The photoluminescence (PL) quantum yield and PL decay time were measured for monolayer MoS2, MoSe2, WS2, and WSe2 at room temperature. Effective exciton radiative lifetimes of > 10 ns were determined from the PL quantum yield of 10(-2) to 10(-3) and the PL decay time of several hundred picoseconds. The results are explained on the basis of the long radiative lifetime of bright excitons at a low temperature limit, a finite exciton coherence area of several square nanometers, and the population of dark exciton states. (C) 2018 The Japan Society of Applied Physics