The mechanical behavior of propylene pentene isotactic copolymers synthesized with a single-center organometallic catalyst in a broad interval of pentene (Pe) concentration, from 3 to 50 mol %, is analyzed. All copolymers show flexible behavior with very high deformation at breaking. The presence and entrance of Pe counits in a and 6 forms of isotactic polypropylene (iPP) transform a strong and fragile iPP homopolymer into ductile and flexible materials. The stress at yield decreases as the Pe content increases, but a different behavior has been observed for samples in the a form or in the 6 form. In samples in the a form, from 3 to 8.8 mol % of Pe, the stress at yield increases as the thickness of crystalline lamellae increases, while in samples in the 6 form containing higher Pe counits, it decreases as the thickness of lamellae increases and Pe content increases. Phase transition of the alpha form in the mesophase and crystallization of the 6 form, in samples with 8-11 mol % of Pe, occur upon deformation and are correlated to the strain-hardening observed at high strain.