We describe the precision synthesis and self-assembly of a series of Janus star polymers with mixed [3:5] heteroarms of polystyrene (PS) and polydimethylsiloxane (PDMS) precisely arranged on a cubic scaffold of T8 polyhedral oligomeric silsesquioxane (POSS) in a spatially segregated fashion. The synthesis begins with a Janus POSS compound with three 2-hydroxylethyl groups on one face and five vinyl pendant groups on the rest vertexes. Facilitated by esterification and a click adaptor, two types of polymer arms are attached efficiently by sequential click reactions, which constitutes a general approach for modular synthesis of such Janus star polymers. The Janus feature is thoroughly characterized using NMR, FT-IR, SEC, and MALDI-TOF mass spectrometry, and the self-assembled structures are studied by small-angle X-ray scattering (SAXS) experiments. With increasing molecular weight of PS arm, the self-assembled phase transits from lamellae (LAM) to hexagonally packed cylinder (HEX) and further to inversed HEX structure, even though the molecular weight of each PDMS arm is as small as 1.4 kDa. Correspondingly, the feature sizes of these samples are very small. For example, the intercolumnar distance is only similar to 6.4 nm in the HEX phase, and the column radius is as small as 1.8 nm. It is anticipated that diverse nanostructures can be created from the self-assembly of these Janus star polymers.