The delay time for primary crystallization plays an important role in the understanding of the transport and nucleation behaviors during crystallization in metallic glasses. In this work, a novel method via Flash DSC is demonstrated to measure the delay time for primary crystallization in Al-based metallic glasses. In a typical Al based metallic glass of Al87Y7Fe6, the delay times were established corresponding to different temperatures. Transmission electron microscopy tests before and after delay time were conducted to further verify the validity of the measured delay time. The effect of a 1 at.% substitution by Cu on the delay time of Al88Y7Fe5 was investigated to determine the effect of minor alloying on the glass formation ability. The significant changes in the primary crystallization temperatures were demonstrated to be directly related to the corresponding changes in the delay time. The results indicate that minor alloying that is designed to increase the delay time is an effective strategy to enhance the glass formation ability.