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In this talk, we report our recent studies on the long-time asymptotics of the Sawada-Kotera (SK) equation and Kaup-Kupershmidt (KK) equation. Both SK equation and KK equation are integrable systems with third-order Lax operator. Moreover, they are related with the same modified nonlinear equation (called modified SK-KK equation) by Miura transformations. This work first constructs the Riemann-Hilbert problem associated with the SK equation, KK equation and modified SK-KK equation by direct and inverse scattering transforms. Then the long-time asymptotics of these equations are studied based on Deift-Zhou steepest-descent method for Riemann-Hilbert problem. Finally, it is shown that the asymptotic solutions match very well with the results of direct numerical simulations. This is a joint work with Xiaodong Zhu.