The electron affinities and low-lying excited states of all the 3d transition metal monoxide molecules are studied using the density functional theory (DFT) and time-dependent (TD) DFT method. The calculated results are compared with the available theoretical ones and used to assign the features of these monoxides in photoelectron spectroscopies. It shows that TDDFT, by and large, can be used to get good results for the excited states of the open-shell transition metal oxides. The effect of basis sets on the calculated results is also discussed.