Polymer tracer diffusion in multiwall carbon nanotube (MWCNT)/Polymer nanocomposites is reported As previously reported for SWCNT/polystyrene (PS) nanocomposites, the tracer diffusion of 680k deuterated polystyrene (dPS) is strongly suppressed at low MWCNT concentrations and then increases at higher concentrations In contrast, the tracer diffusion of 10k dPS and 75k dPS is Independent of MWCNT loading These results reveal all important criterion for exhibiting a minimum in the tracer diffusion coefficient (D-min) with nanoparticle concentration, namely the relative size of tracer molecule and nanoparticle. Specifically, when the radius of gyration oft lie tracer polymer(R-g) is smaller than the radius of the nanotube particle (R-CNT), the tracer diffusion is independent of nanoparticle concentration, while a D-min is observed when R-g > R-CNT When the tracer molecule is large relative to the nanoparticle, the diffusion in the vicinity of the nanoparticle appears to become anisotropic, which leads to a D-num with Increasing nanoparticle le loading.