We focus on chain organization of long and short block chains on lamellar microdomains developed in a series of binary mixtures denoted as as/s(i), which are composed of a long asymmetric polystyrene-block-polyisoprene (SI) and one of three short symmetric SI's denoted respectively as as and s(i) (i = 1, 2, 3). We investigated the effect of chain organization on stability of the microdomain and on microdomain size as a function of blending composition and ratio of degree of polymerization of as to si. Comparisons of the results with some theoretical predictions indicated that the strong segregation theory of Birshtein, Zhulina, and Lyatskaya fits well with the experimental results.