Background/Aims A light‐cured intermediate material is useful for fabricating a hard insert and a buffer space mouthguard (H&SMG). However, it requires improvement in its mechanical properties and shock‐absorbing capacity. The aim of this study was to evaluate the mechanical properties of two prototype light‐cured intermediate materials reinforced with glass fibers, and the impact absorption capacity and durability of H&SMGs made with the prototype intermediate materials. Materials and Methods Two prototype materials containing long and microlength glass fibers in a light‐cured intermediate material, Innerframe LC®, for H&SMG, were fabricated and tested. A three‐point bending test was performed for evaluation of the mechanical properties. In addition, a shock absorption test was conducted using a customized pendulum impact testing machine to evaluate the H&SMGs' impact absorption capacity and durability. Results Long and microlength glass fibers significantly improved flexural modulus and strength. H&SMGs made with these two glass fiber‐containing materials had high impact absorption capacity against both low and high impact forces, while the mouthguards made with long glass fiber materials had the best results. Conclusion Long and microlength glass fibers with the prototype materials improved the mechanical properties of Innerframe LC® and the impact absorption capacity and durability of H&SMGs. H&SMGs made with the long glass fiber prototype materials had the best performance.