The effect of fillers and the structure of matrix rubbers on the tensile stress -strain behavior were examined with unvulcanized compounds of cis-1,4-polybutadiene. The matrix rubbers were CB11, CB22, and VCR412, which had been studied previously. The fillers were two different carbon blacks ( N110 and N330 ) and silica. The tensile modulus was found to be separable into the time-dependent and strain-dependent functions. The time dependence was represented with a universal form, the observed time being reduced with the extension ratio. The strain-dependent function was affected by material variables. The function indicated a strainsoftening tendency. The extent of softening is related to the softening or hardening observed with the unfilled matrix rubbers, which are in turn related to the length and degree of branching and to the presence of crystalline particles of 1,2-polybutadiene. However, carbon black evidently accentuates the softening tendency. Also, poorer dispersion results in higher softening. The silica-filled compound had a poor dispersion. With the addition of a coupling agent, the dispersion improved but the compound behaved like it was crosslinked. The strain-induced crystallization previously observed with CB11 and VCR412 gum rubbers ( but not with CB22 ) appears as significant differences of the moduli of the compounds.