The intrinsic viscosity [eta] was determined for 25 samples of atactic oligo- and poly(alpha -methylstyrene)s (a-P alpha MS), each with the fraction of racemic diads f(r) = 0.72, in the range of weight-average molecular weight M-w from 2.94 x 10(2) (dimer) to 3.22 x 10(6) in cyclohexane at 30.5 degreesC (Theta). The translational diffusion coefficient D was also determined from dynamic light-scattering measurements for 21 of them in the range of M-w from 5.30 x 10(2) (tetramer) to 3.22 x 10(6) under the same solvent condition, It is found that the double-logarithmic plots of [eta] and MwD against M-w follow their respective asymptotic straight lines of slope 1/2 for M-2 greater than or similar to 2 x 10(5), but deviate upward and downward, respectively, from them with decreasing M-w for smaller M-w. From an analysis of these transport coefficients on the basis of the helical wormlike (HW) chain model, it is shown that the above M-w dependence of [eta] and D may be well explained by the corresponding HW theories with the values of the model parameters consistent with those previously determined from the mean-square radius of gyration [S-2]. A comparison is also made of the present results for [eta] and D for a-P alpha MS with the previous ones for atactic polystyrene with f(r) = 0.59, atactic poly(methyl methacrylate) (a-PMMA) with f(r) = 0.79, and isotactic PMMA with f(r) similar or equal to 0.01. It confirms the previous conclusion derived from (S2) concerning the chain stiffness and local chain conformations of the a-P alpha MS chain. That is, the a-P alpha MS chain tends to retain large and clearly distinguishable helical portions in dilute solution just as the a-PMMA chain, although the strength of helical nature is somewhat smaller for the former.