Synthesis of hybrid hetero-arms star polymer with precisely defined arms distribution in three dimensional (3D) space is still a challenge. Herein, we have employed cubic polyhedral oligomeric silsesquioxane (POSS) nanoparticle as a scaffold to synthesize a biodegradable Janus POSS star polymer (oom-T(8)PLLA(5)PCL(3)), which contains an inorganic rigid POSS core, three poly(epsilon-caprolactone) (PCL) arms and five poly(L-lactide) ( PLLA) arms on the POSS cage two sides, respectively. The chemical structure of oom-T(8)PLLA(5)PCL(3) was confirmed by NMR, SEC, and FTIR spectra. As an example of application for the Janus POSS star polymer, oom-T(8)PLLA(5)PCL(3) was used for the compatibilization of biodegradable PLLA/PCL (w/w = 70/30) blends. It was found that oom-T(8)PLLA(5)PCL(3) could reduce the dispersed PCL phase size in PLLA matrix and enhance the properties of PLLA/PCL polymer blends, which was reflected in the increase of Izod impact strength and tensile performance of the polymer blends. (c) 2017 Elsevier Ltd. All rights reserved.