Weight reduction is a priority across the automotive industry for improved fuel efficiency and CO2 reduction, and crash safety is the first design criterion considered during vehicle development. This paper is concerned with lightweight design of front rail on a vehicle chassis frame structure through material substitution, i.e., replacing the original mild steel with advanced high strength steel (AHSS), without deteriorating the vehicle’s crashworthiness performance. Magic Cube approach (MQ), a systematic design approach, is conducted to analyze the design problem: by applying space decomposition, a system-to-subsystem-to-component model is developed, which simplifies the design problem. Numerical simulation is carried out with LS-DYNA to evaluate the crashworthiness performances of the original and the new-designed front rail. The result shows that up to 30% weight can be reduced in the new design through material substitution while the crash performance remains the same as in the original design.