The mean-square radius of gyration [S2] was determined by small-angle X-ray scattering and light scattering for 16 samples of isotactic oligo- and poly(methyl methacrylate)s (i-PMMA) in the range of weight-average molecular weight M(w) from 4.58 x 10(2) (tetramer) to 1.71 x 10(6) in acetonitrile at 28.0-degrees-C (theta). The fraction of racemic diads f(r) determined for them by H-1 NMR was ca. 0.01 independently of M(w) except for the oligomers with very small M(w), for which it was somewhat larger. In contrast to the previous results for atactic (a-) PMMA with f(r) = 0.79 in the unperturbed THETA state, for which the ratio [S2]/x(w) as a function of the weight-average degree of polymerization x(w) exhibits a maximum, it was found to increase monotonically with increasing x(w) and level off to its asymptotic value for i-PMMA as in the case of atactic polystyrene (a-PS) with f(r) = 0.59 previously studied. The data obtained were analyzed on the basis of the helical wormlike (HW) chain model to determine the model parameters. The results are lambda-1kappa0 = 2.5, lambda-1tau0 = 1.3, lambda-1 = 38.0 angstrom, and M(L) = 32.5 angstrom-1, where kappa0 and tau0 are the differential-geometrical curvature and torsion, respectively, of the characteristic helix taken at the minimum zero of the elastic energy of the model, lambda-1 is the stiffness parameter, and M(L) is the shift factor. The difference in local chain conformation between i- and a-PMMA is discussed in detail on the basis of the model parameters thus determined. It is then concluded that the above difference in the behavior of [S2]/x(w) between i- and a-PMMA arises from the fact that the former is of weaker helical nature than the latter. In order to illustrate the situation, a picture is given of representative instantaneous contours of HW Monte Carlo chains for them and also for a-PS.