The characteristics of double taper shaped beam of the fibre reinforced plastic materials was analysed. The optimal taper ratio was proved to be 1/2, and the effect of fibre off-axis in the tapered region on the beam stiffness was negligible. On the basis of this, the longitudinal type, asymmetric, double tapered GRP leaf spring was designed for replacement of the four leaf steel spring in the rear suspension system of small passenger car. The 2-D and 3-D stress analysis showed that 11 is critical and that the unidirectional structure is optimal. Two kinds of prototype were fabricated with E-glass/epoxy and S2-glass/epoxy, using the dynamic dielectric analysis to optimize the curing process. The a.c field of $\approx 10 KHz$ was suitable for the monitoring of the changes of dielectric constant during the early stage of the curing process. The weight saving of approximately 50% was possible with the prototype GRP leaf spring compared to the conventional steel spring of the same spring constant (2.3Kg/mm). The prototypes had the fatigue life more than 5 times than the steel spring under $265Kg \pm 70mm$ cyclic load without hysteresis, and had fail safe characteristics. The stress wave factor measurement with the acousto-ultrasonic method during the road test of the prototypes indicated that the damage was not initiated during the 14,000 Km road test.