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Abstract: A class of ultra low dielectric constant polymethylsilsesquioxane (PMSQ) films with T8(Me4NO)8 polyhedral oligomeric silsesquioxanes (T8 POSS) as double-effective porogen was studied. Through the Me4NO− groups of T8 POSS attacking the Si–O–Si chains of PMSQ, the POSS will be connected to the PMSQ crosslink system. POSS has the cage structure, which acted as the closed pores (≤1.5nm). On the other hand, the Me4NO− groups served as the sacrificial template. When they decomposed after annealing, the open pores were then left in the films. The introduction of T8 POSS can greatly decrease the dielectric constant of PMSQ, and effectively improve its mechanical strength owing to T8(Me4NO)8 interconnected with PMSQ. These continuous and smooth films were prepared by spin-coating with thickness in the range of 60–200nm. The dielectric constant of the films could be controlled by adjusting the proportion of porogen. These films showed good mechanical strength and ultra low dielectric constant. In particular, a POSS/PMSQ film with ultra low dielectric constant of 1.6 and modulus of 7GPa had been prepared on silicon wafer by spin-coating. [Copyright &y& Elsevier]