It is one of the goals for science researchers to improve the glass forming ability (GFA), overcome the limit to dimensions of metallic glasses and search for a simple and reliable gauge for quantifying the GFA of metallic glasses. In the work, five kinds of Zr based BMGs with different GFA, which were Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5、Zr₅₅Al₁₀Ni₅Cu₃₀、Zr₄₅Cu₄₅Ag₅Al₅、Zr₆₅Al_7.5Ni₁₀Cu_17.5、Zr₅₇Ti₅Al₁₀Cu₂₀Ni₈ respectively, were used to study the relationship between GFA and internal friction (IF). By comparing the internal friction-temperature curves of different samples, it is found that the higher the value of IF peak is, the better the GFA is. Compared with the activity energy evaluated during the supercooled liquid, the nature of internal friction shows the reason of metallic glass with high GFA. So a new criterion evaluating the GFA is proposed, i.e. internal friction peak. Besides, based on the internal friction-loading frequency curves of five samples, the behavior of mechanical relaxation was discussed. It is showed that the more GFA is, the more the number of instability atoms in the supercooled liquid is, the shorter the time of mechanical relaxation is.