The quantitative parameters such as supercooled liquid region or several reduced glass transition temperatures were applied for evaluating the glass-forming ability of Pd-based metallic glasses. A distinct proportional tendency was recognized between measured critical cooling rates and reduced glass transition temperature by liquidus temperature rather than eutectic temperature. Significance and physical meanings of the quantitative parameters for evaluating of glass-forming ability were also discussed. Furthermore, a new concept of a modified reduced glass transition temperature was proposed to evaluate the glass-forming ability. The modified reduced glass transition temperature as a function of critical cooling rate was found to exhibit a much clear linearity.