Ever since Sterman and Weinberg calculated two-jet cross section of $e^+e^-$ annhilation in asymptotically free Quantum chromodynamic (QCD) perturbation theory, many theoretical works on the $e^+e^-$ jet structure have been done in the context of QCD. In this article, we study the basic theorem on mass singularities in Quantum electrodynamics (QED) and see how it is applied to $e^+e^-$ two-jet process in perturbative QCD. And we calculate hard gluon bremstahlung process which may give rise to deviations from two-jet structure. This will be described by reliable jet variables such as thrust and spherocity. Finally, we give $e^+e^-$ annihilation energy pattern predicted in perturbative QCD.