Although the thermal conductivities of MoS2 and WS2 have been reported by some experimental and theoretical studies, the results are inconsistent. Here, thermal transport properties of twelve types of single layer transition metal dichalcogenides (TMDs) MX2 (M = Cr, Mo, W; X = 0, S, Se, Te) are investigated systematically, by solving Boltzmann transport equation based on first-principle calculations. After accurate considering the size effect and boundary scattering, we find that our calculations can fit the experimental results very well. Moreover, diverse transport properties in TMDs are revealed, and an abnormal dependence of thermal conductivity on atomic mass is observed. In most MX2 structures, the thermal conductivities decrease with the increase of mass of atom M or X. However, the thermal conductivities of sulfides MS2 and selenides MSe2 increase as the M changes from Cr to Mo to W, which is contradictory to our traditional understanding. A detailed calculation indicates that the abnormal trend is originated from the rapid increase of phonon relaxation time. Our studies provide important information for thermal transport abilities in TMDs. (C) 2016 Elsevier Ltd. All rights reserved.