We conducted molecular dynamics. (MD) simulations to study how solvent chains affect the diffusion of linear polymers in the unentangled regime. For mono disperse solvent chains, the self-diffusivity of a tagged chain scales with its chain length. The solvent chain length affects both the prefactor and the exponent, the latter of which ranges from -0.79 to -0.85. The scaling exponent here deviates from-1 as predicted by the Rouse model, which may suggest that the friction coefficient increases with the solvent Chain length. In addition, we carried out diffusion simulations on two polydisperse melts, one with the Flory-Schulz distribution and the other with the Gaussian distribution. The measured diffusivity as a function of the tagged chain length agrees with a simple proposed model accounting for the heterogeneous medium.