Using density functional theory calculations, molecular g tensors are theoretically investigated for rubrene and oligoacenes. The anisotropy in the calculated g shifts of rubrene well explains experimental results observed by field-induced electron spin resonance (FI-ESR) measurements. We demonstrate that the combination of the FI-ESR technique and the calculation of molecular g tensor is a quite useful approach to determine the molecular orientation at interfaces in organic field-effect transistors.