The diffusion process of poly(diethylsiloxane) (PDES) has been studied by field-gradient NMR at temperatures from 20 to 80 degreesC, which is recognized to take the biphasic phase consisting of the liquid crystalline and the isotropic regions at temperatures from 20 to 40 degreesC and the isotropic phase at temperatures from 50 to 80 degreesC by solid-state Si-29 NMR. The diffusion coefficients (D) of PDES in the liquid crystalline region of the biphasic phase have been measured at 20 and 30 degreesC by pulse field-gradient stimulated-echo (PGSE) C-13 NMR with H-1 CW decoupling as designed by us, and also those in the isotropic region of the biphasic phase and in the isotropic phase have been determined by conventional PGSE H-1 NMR. From these experimental results, it is found that the diffusion coefficient of PDES in the liquid crystalline region is much larger than that for the PDES in the isotropic region. This shows that the smaller diffusion coefficient of PDES chains in the isotropic region (approximate to 10(-10)-10(-11) cm(2)/s) compared with that in the liquid crystalline region (approximate to 10(-8) cm(2)/S) comes from the entanglements effect between the polymer chains.