Drug delivery from polymer nanocarriers is usually achieved by designing polymers so that they release drugs by cleavage of labile bonds, or by preparing nanoparticles that swell or collapse in response to external stimuli. Herein, we unravel the importance of polymer crystallinity in release profiles of drugs encapsulated in polymer nanoparticles. Polycaprolactone, as a model biocompatible and biodegradable semi-crystalline polymer, was processed into nanoparticles by the miniemulsion-solvent evaporation technique. The crystallinity of the nanoparticles was controlled by the polymer concentration, size of nanoparticles, and the composition of mixtures with amorphous polymers such as poly(vinyl formal) and polystyrene. Crystallinity decreased significantly with decreasing nanoparticle diameter. Release profiles of drugs were found to be dependent on an interplay of nanoparticle size and crystallinity. Therefore, crystallinity can be used for tuning the release profiles of nanoparticles. (C) 2019 Elsevier Inc. All rights reserved.