A novel diblock copolymer, poly(ethylene oxide-b-sebacic acid) (PEO-b-PSA), was prepared by polycondensation. Its self-assembly in water via a microphase inversion resulted in narrowly distributed stable polymeric nanoparticles with a size of similar to 70 nm. Such formed nanoparticles had a core-shell nanostructure: with the insoluble hydrophobic PSA blocks as the core and the soluble hydrophilic PEO blocks as the protective shell. The core was degradable, and its degradation led to the disintegration of the nanoparticle. The structure of the nanostructure and the degradation kinetics were investigated by a combination of static and dynamic laser light scattering. Our results indicated that the degradation was a first-order reaction, and the degradation rate increases with the dispersion temperature.