Vibrational spectra of 2-methyl-3-nitrophenyl isocyanate and 4-methyl-2-nitrophenyl isocyanate, in the spectral region 4000-100 cm(-1), have been measured and assigned. Conformational and harmonic frequency analyses have been performed at B3LYP/6-311G(∗) level of calculations. The two stable conformers, cis and trans, have been computed for each of the molecules. It has been determined that the trans conformer has lower energy than the cis by 3.954 kJ/mol for 2-methyl-3-nitrophenyl isocyanate; whereas the cis conformer has lower energy than the trans by 10.230 kJ/mol for 4-methyl-2-nitrophenyl isocyanate. The vibration structure of 2-methyl-3-nitrophenyl isocyanate conforms to the combined behavior of its both conformers from which the deviation is shown by the structure of 4-methyl-2-nitrophenyl isocyanate which follows only the trans conformer. The occurrence of symmetric mode of the methyl group at higher frequency near 2944-20 cm(-1) is attributed to the phenyl ring strain caused by the substituents. As for the other stretching and bending modes, mutually exclusive pattern appears to work for the molecules: The nitro group's non-coplanarity with the phenyl ring is more evident in 4-methyl-2-nitrophenyl isocyanate where the asymmetric mode was assigned to the band at 1569cm(-1), whereas the symmetric mode at lower frequency 1339cm(-1). Occasional doublet appearance of the strong asymmetric absorption near 2282cm(-1) due to isocyanate moiety has been observed in the present study and is assumed to arise from the torsional vibration motion of the moiety rendered by the small energy gap between the conformers of 2-methyl-3-nitrophenyl isocyanate.