The diffusion coefficients of poly(beta-benzyl L-aspartate)(PBLA) with the alpha-helix (rodlike) form in chloroform and the random-coil form in a mixture of chloroform and trifluoroacetic acid have been measured as a function of the PBLA concentration (C-PBLA) by using high field-gradient H-1 NMR. The PBLA concentration range for the former is from 0.11 to 9.20 wt % and for the latter it is from 0.20 to 25.2 wt %. From these experimental results, it is found that the diffusion coefficient of PBLA in the rodlike form is much smaller than that of PBLA in the random-coil form at the same PBLA concentration. This implies that the small diffusion coefficients of PBLA in the rodlike form as compared with those in the random-coil form come from the relatively large radius of gyration R-g. Further, it is found that the diffusional behavior of PBLA in the rodlike form is different in the three PBLA concentration regions, that is, region I (C-PBLA = 0.110 similar to 0.39 wt %). region 2 (C-PBLA = 0.39 similar to 0.86 wt %). and region 3 (C-PBLA = 0.86 similar to 9.20 wt %). In the random-coil form, the diffusion is different in the two PBLA region II (C-PBLA = 1.03 similar to 25.2 wt concentration regions, that is, region I (C-PBLA = 0.20 similar to 1.03 wt %) and region These diffusional behaviors in the rodlike form and in the random-coil form can be reasonably explained by Tinland-Maret-Rinaudo theory and de Gennes theory, respectively. Further, transitional change from the rodlike form to the random-coil form is discussed on the basis of the diffusional behavior.