The influence of beta crystals on the mechanical properties of isotactic polypropylene is studied on compression-moulded sheets without filler or filled with stearate-coated calcium carbonate particles. A crystallisation procedure is setup for producing sheets with various amounts of beta crystals, trying to keep constant crystallinity, spherulite size and crystal thickness. An optimum of beta crystals among alpha crystals is first produced by isothermal crystallisation at 110 degreesC. Then gradual transformation of beta into alpha. crystals is carried out through thermal annealing at 152 degreesC after an intermediate cooling down to room temperature. Due to the thermal inertia of the large sheet thickness, the amount of beta crystals does exceed 10% for unfilled sheets and 50% for filled sheets. Dynamic mechanical behaviour shows that molecular mobility is higher in the beta crystals than in the alpha crystals, as judged from the temperature of the crystalline relaxation. Loss modulus in the temperature range of the crystalline relaxation also indicates greater damping capacity for the beta crystals. This is discussed in terms of activation of conformational defects moving along the chain stems in the crystal. Plane strain compressive testing reveals better ductility for beta rich samples. Interpretation is provided using an approach of semi-crystalline polymer plasticity based on dislocation-governed crystallographic slip. Correlation is made with the viscoelastic behaviour through the concept of conformational defects.