The characteristics of gate-modulated generation (GMG) current IGMG in nano-scale LDD nMOSFET under the reverse substrate bias VB are investigated. It is found that the rising and falling edges of IGMG curve shift rightwards as |VB| increases. On the basis of experimental and theoretical analysis, the physical mechanism behind this shift phenomenon is attained. The shift phenomenon is ascribed from the fact that VB modulates the proportion of surface potential φs in the gate bias VG. φs decreases with |VB| increasing under a certain VG, and consequently the maximum generation factor of the rising edge (gr) diminishes and that of the falling edge (gf) augments. Further, it is found that the transconductance peaks of the rising edge (GMR) and falling edge (GMF) increase with |VB| increasing. Moreover, GMR and GMF both have the linear relationship with VB in log coordinate. Due to the different roles of drain bias VD on the rising and falling edge of IGMG curve, GMR keeps constant but GMF varies under different values of VD. Increasing VD can enhance the change of gf with VG, there by increasing GMF under a given VB. Also, this results in the fact that the trend of GMF increasing with |VB| increasing slows down under a larger VD: the slop of GMF-VB curve decreases from 0.09 to 0.03 as VD increases from 0.2 to 0.6 V.