RMnAsO (R = Ce and La) are isostructural compounds with Fe-based oxypnictides, but has a unique properties as Mott insulator unlike the nonsuperconducting parent compounds of the Fe pnictides that are metallic. Also, our preliminary inelastic experiment at DCS, NIST shows that only a single non-dispersive magnetic excitation exists upto 20 meV, which is puzzling because Fe-base 1111 and 122 compounds show a broad spin wave bandwidth ~ 200 meV with gap energy ~ 8 meV from the long range antiferromagnetic order of Fe ions.We would like to propose time-of-flight neutron scattering experiments on CeMnAsO and LaMnAsO at ISIS where the range of excitation energy can be covered up to 200 meV which is suitable for investigating high-energy spin excitations expected in these systems.