The user has requested enhancement of the downloaded file. arXiv:cond-mat/0508539v1 [cond-mat.str-el] We report 121,123 Sb nuclear quadrupole resonance (NQR) measurements in the filledskutterudite superconductor PrOs 4 Sb 12 in the temperature range of 0.05-30 K. The electric field gradients (EFG), V zz and V xx − V yy , at the Sb site exhibit unusual temperature dependence below 30 K. To explain these features, we discuss the coupling between the Sb nuclear quadrupole moment and Pr 4f 2 -derived multipole moments. The observed T dependence of EFG is well explained by the CEF quasi-quartet consisted of Γ 1 singlet and Γ