Transparent glass fiber composites were prepared with a refractive-index-adjustable thermoplastic blend. The opacity of a polymer blend is affected by the phase separation and crystallization properties. Polyamide 6 (PA6)/poly ( m -xylene adipamide) (MXD6) blends are intrinsically miscible and transparent in the molten state; further, crystallization is suppressed by polymer chain disordering induced by in-situ transamidation. Thus, the transparency of the molten state was maintained after a sufficient period of melt-mixing. The refractive index was adjusted to that of the glass fiber reinforcement by altering the PA6/MXD6 blend ratio. Injection-molded glass fiber composites of PA6/MXD6 had good transparency and mechanical properties. • Injection moldable transparent thermoplastic composites are developed. • Miscible polyamide blends are utilized as refractive index adjustable matrix. • Crystallization of polyamide blends is eliminated by in situ transamination. • Refractive index matching is achieved by adjusting weight fraction of polyamides. • The transparency and mechanical properties are better than those of PC composites.