An arbitrary perturbation term in the quantized field theory is expressed explicitly in terms of generalized Feynman parametric integral, after all integrations of momenta are carried out. By means of this general integral formula Dyson's power-counting theorem as well as the absence of odd order divergences are proved rigorously, and Lehmann's spectral function ρ(u) is given explicitly in perturbation expansion. Some remarks are made on the convergence of perturbation series.