© 2020 Glass forming ability (GFA) is critically important for large-scale applications of bulk metallic glasses with maximized functionalities at low costs in materials and processing. Traditionally, excellent glass formers usually have compositions in eutectic regions where liquid phases have high stability and low liquid temperatures while GFA of off-eutectic alloys is largely limited by primary crystallization. In this study we reported that the GFA of Ni-based off-eutectic alloys can be dramatically improved by manipulating the primary crystallization reaction through minor alloying. As a result, a novel Ni60Pd20P14Si2B4 metallic glass with excellent GFA is developed by substituting P in Ni60Pd20P20 using 2 at.% Si and 4 at.% B. The minor alloying leads to the transition of the primary crystallization from a tetragonal (Ni, Pd)3P phase to a chemically modulated complex monoclinic phase. The enhanced GFA results in the largest achievable processing diameter (25 mm) of Ni-based bulk metallic glasses, which is more than four times larger than that of Ni60Pd20P20. The complex crystal structure of the primary crystalline phase with chemical modulation and a large unit cell leads to slow crystal growth, and hence the improved stability of supercooled liquid for better GFA.