The aim of the present study was to characterize pegylated nanoparticles (NPs) for their microporosity and study the effect of microporosity on drug release kinetics. Blank and drug-loaded NPs were prepared from three different pegylated polymers, namely, poly(ethylene glycol)1%-graft-poly(D,L)-lactide, poly(ethylene glycol)5%-graft-poly(D,L)-lactide, and the multiblock copolymer (poly(D,L)-lactide-block-poly(ethylene glycol)-block-poly(D,L)-lactide)n. These NPs were characterized for their microporosity using nitrogen adsorption isotherms. NPs of the multiblock copolymer showed the least microporosity and Brunauer-Emmett-Teller (BET) surface area, and that of PEG1%-g-PLA showed the maximum. Based on these results, the structural organization of poly(D,L)-lactide (PLA) and poly(ethylene glycol) (PEG) chains inside the NPs was proposed and was validated with differential scanning calorimetry (DSC) and X-ray photoelectron spectroscopy (XPS) surface analysis. An in vitro drug release study revealed that PEG1%-g-PLA NPs exhibited slower release despite their higher surface area and microporosity. This was attributed to the presence of increased microporosity forming tortuous internal structures, thereby hindering drug diffusion from the matrix. Thus, it was concluded that the microporous structure of NPs, which is affected by the molecular architecture of polymers, determines the release rate of the encapsulated drug.