The microstructure evolution of laser rapidly solidified Pd77Cu6Si17 alloy was investigated. The experimental results showed that with increasing growth velocity, a phase transition series of Pd3Si dendrite+eutectic-ξ dendrite+eutectic-eutectic grain-regular eutectic-amorphous phase occurred. The critical velocity of amorphous transition was determined to be about 6 mm/s and the smallest lamellar spacing was 35 nm. Compared with those of Al-25 wt% Sm and Al-32.7 wt% Cu alloys, it was proposed that low diffusion coefficient is the main reason for the good glass formation ability of Pd77Cu6Si17 alloy.