Photoisomerization of acetyl methyl iminoxy radical produced from NO2 and 2-butyne in low-temperature argon matrices has been studied by density functional theory (DFT) calculation. The infrared bands observed upon 585 nm irradiation are assigned, using the spectral patterns predicted by DFT, to the trans-trans, cis-trans, and cis-cis conformers, where the former and latter conformations refer to the CO-CN and C=N bond axes, respectively. Conformational isomerization around the C=N bond is found to occur upon 515 nm irradiation. The isomerization mechanism is discussed with the aid of the DFT calculation.