  We present a rotation average of the two-body scattering amplitude in the
lightcone time($\tau$)-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 $\phi^{3}$ theory, we find that the higher Fock-state
contribution is quite small in the lightcone quantization.
