The mechanical behavior of a polyamide 12 is characterized using a viscoplastlc law taking into account viscoelastic deformation. Experimental determinations are performed at constant and very low strain-rates in isothermal conditions during tensile tests. A finite elements code, based on this behavior, enables us to reproduce tensile tests even at different strain rates (up to 0.5 s(-1)) provided that heat power dissipation due to deformation is taken into account. The same model partially reproduces multiaxial impact tests. Strain hardening is generally over-estimated by the calculation. It is pointed out that mechanical behavior should be characterized in ways closer to impact loading conditions than tensile tests to be more efficient because of the difference in microstructural evolutions during drawing.