This paper attempts to obtain insight about the effect of modal interaction of higher than 2nd order in small-signal analysis, through evaluating the effect of 3rd order modal interaction. The indices that quantify the 3rd order modal interaction are defined, following the modal analysis procedures used in linear modal analysis and second order modal analysis. Numerical simulation results demonstrates that, there are definitely modal interaction higher than 2nd order, and with further higher order modal analysis, it is possible to detect more information about modal interaction, which might be useful in power system control.