Due to alternative exchange between single and double-teeth meshings, the upper part of gear tooth is subjected to dynamic tensile stress whose growth rate presents rectangular fluctuation. This work investigates the atomistic scale behaviors of intergranular crack propagation along twist grain boundary in body-centered cubic (bcc) iron under dynamic tensile stress. The effects of driving force and contact ratio are fully discussed. Results show that only stacking faults with face-centered cubic (fcc) atoms can be formed in lower monocrystal portion. Edge dislocations in upper monocrystal portion are suppressed by intergranular crack cleavage. Critical stresses for stacking fault nucleation and intergranular crack cleavage vary with driving force and contact ratio. By calculating actual stress intensity factor at crack tip, variations of critical stresses are found to be attributed to the variations of time-dependent factors. Although critical stresses vary with driving force and contact ratio, the effects of these factors on the growths of crack length and plastic zone are not obvious in the early stage of intergranular crack propagation. Accumulated plastic strain energy before intergranular crack cleavage is independent of driving force and contact ratio. Departing from the early stage, the growth rates of crack length and plastic zone increase significantly with an increase in driving force or a decrease in contact ratio. However, the final ductile level of intergranular crack propagation cannot vary with contact ratio and large driving force. By applying dynamic load, this work can be used to reveal the atomistic scale mechanism of gear failure. The results can provide a good reference for gear safety design.