From the requirements of analyticity and unitarity, the resonance behaviours of the partial wave amplitudes for π-π scattering are discussed in general. The theory involves a function F(l)I)(v), which indicates deviation from the Breit-Wigner type resonance formula. When F(l)I)(v)=1, the amplitude has exactly the Breit-Wigner form. Further investigation shows that the function F(l)I)(v) in general deviates from 1. By use of the ND-1 method, the crossing symmetry and the ρ- and f0-bootstrap approximation, by comparing with the integral expression of the partial wave amplitude for the interval -90 are calculated. The method takes into account both the contributions of the inelastic process and the dispersion integral of the negative interval. The calculation gives v(R1)=6.4,Г1=0.12 for the J=1, I=1 state and v(R2)=20, Г2=0.016 for the J=2, I=0 state. These results correspond to a mass of 762 MeV and a half-width of about 45 MeV for the ρ meson, and a mass of 1283 MeV for the f0 meson. They are in good agreement with recent experimental data.