In this study, we designed a polymersome system for the controlled release of methotrexate (MTX) as an anticancer drug with the objective of improving the loading efficiency of the drug in polymersomes as well as achievement of an efficient control on the release rate of drug from nanocarriers. We synthesized mono methoxy poly(ethylene glycol)-poly(e-caprolactone) (mPEG-PCL) diblock copolymers. The structure of the copolymers was characterized by proton nuclear magnetic resonance spectroscopy (H NMR), Fourier transform infrared spectroscopy (FT-IR), and differential scanning calorimetry (DSC) techniques. MTX was encapsulated within nanoparticles (NPs) through multiple emulsion method. The resulting NPs were characterized further by various techniques such as atomic force microscopy (AFM) and dynamic light scattering (DLS). Next, the various kinetic equations were fitted to the release data of MTX from MTX-loaded mPEG-PCL polymersomes. The results showed that the zeta potential of MTX-loaded mPEG-PCL polymersomes was about -5.49 mV and the average size was 49.18 nm. MTX was encapsulated into polymersomes loading capacity of 12 ± 0.09% and encapsulation efficiency of 45.5 ± 0.41%. The metabolic activity assays of void of MTX, mPEG-PCL polymersomes, and MTX-loaded mPEG-PCL polymersomes were compared to each other by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay of the treated MCF-7 cell lines. It can be concluded that application of NPs is a better and more effective strategy for controlled and slow release of MTX in the treatment of cancer.