The properties X (stress, crystallinity, melting temperature, crystallite sizes) of stretched filled and unfilled natural rubber (NR) are compared. The measurements have been done during deformation at equilibrium and at room temperature. From each physical property X in static and dynamic experiments one defines for the filled NR an amplification factor A(X) and a differential amplification factor A*(X). From stress and WAXS measurements two regimes are observed: For lambda < lambda(A), the material has not crystallized and the amplification factor A(X) is independent of lambda. A(X) is on the order of 2 for the sample filled with 20% black carbon (volume concentration). It is shown that the Guth relation fits rather well our data and those of Lee and Donovan on similar filled NRs. For lambda > lambda(A), stress-induced crystallization occurs, A(X) approximate to 1.5-3 is weakly dependent on lambda, and the differential amplification factor is constant. The values of A*(X) determined by the different measurements are of the same order of magnitude. These X amplification factors characterize the state of extension of the amorphous chains during the process of stress-induced crystallization.