We present a rotation average of the two-body scattering amplitude in the lightcone time(τ)-ordered perturbation theory. Using a rotation average procedure, we show that the contribution of individual time-ordered diagram can be quantified in a Lorentz invariant way. The number of time-ordered diagrams can also be reduced by half if the masses of two bodies are same. In the numerical example of ϕ^3 theory, we find that the higher Fock-state contribution is quite small in the lightcone quantization.