Direct numerical simulation of a two-dimensional mixing layer is conducted and mass transport phenomenon are discussed based on the database obtained from the simulation. Retardation of mass transport in the initial vortex formation region of the mixing layer is pointed out and its mechanism is discussed by comparing the terms that make up the transport equation of mass flux <(v＇c＇) over bar>. Retardation of mass transport originates through the effect of the turbulent diffusion term -partial derivative(<(v＇v＇c＇) over bar>)/partial derivative y suppresses the mass transport. This phenomenon is discussed in detail by considering the instantaneous turbulent diffusion term. At the initial vortex formation region of the mixing layer, interaction between (1) large velocity induced by a secondary flow formed by the interaction between the vortices and (2) tail-type instantaneous concentration profile is considered to have a large effect on the instantaneous turbulent diffusion term, retarding mass transport.