Uniform stereo-complex microparticles ranging from nanometer to micrometer size are prepared by using stereo multiblock co-poly(rac-lactide)s (smb-PLAs) with different stereo-regularity. At comparable molecular weights, as the smb-PLA stereo-regularity decreases from 88% to 76%, the crystallinity of the microparticles decreases noticeably, as proved by DSC and WAXD. At the same time, the shape of the microparticles varies from the flower shape to the sphere shape and the particle size increases markedly from 700-2700 nm as shown by SEM. However, all insulin-loaded microparticles are of cake-shape and their sizes depend on the stereo-regularity. The crystallization of smb-PLAs facilitated by insulin is evidenced by the increase of T(m) and DeltaH(f) in DSC. The highest insulin-loading content of 14.2% and -entrapment efficiency of 82.8% are obtained from the smb-PLA with the highest stereo-regularity of 88%. Release studies in vitro show the least first-day release at about 25% followed by continuous release of another 70% of insulin over one month. Stereo-complex microparticles of smb-PLAs with lower stereo-regularity resulted in a relatively lower insulin-entrapment efficiency and -loading content, a larger first-day release, and also complete release of 90% of the total amount within one month. The release system follows a diffusion mechanism. By contrast, atactic PLA shows a very low entrapment efficiency of 16.7%. Structure of a stereo multiblock co-poly(rac-lactide).