Mechanical properties of polymers can be described by their stress/strain curves and by their behavior under dynamic mechanical thermal analysis (DMTA). The purpose of this paper is to report such mechanical properties for two unsaturated polycarbosilanes : poly(1,1-dimethyl-1-sila-cis-pent-3-ene) (I) and poly(1-methyl-1-phenyl- 1-sila-cis-pent-3-ene) (II). Tensile strength, elongation at break, modulus, bending modulus, T-g, and tan delta for I, II and for sulfur crosslinked I and II have been measured. The influence of polymer molecular weight, quantity of crosslinking agent, cure time, presence of carbon black filler, the effect of crosshead speed, and frequency on these properties was investigated.