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Nitrogen-pair formation processes on the GaAs(001)-(2×4) surfaces found experimentally are systematically investigated using ab initio-based approach incorporating beam equivalent pressure p (pAs = 3.0 × 10 −6 Torr, pN = 7.0 × 10 −6 Torr) and temperature T = 830 K. The calculated surface phase diagrams elucidates that the (2×4)α1 and (2×4)α2 surfaces are stable at the growth conditions under As2 and As4, respectively. The Monte Carlo simulations reveal that the N incorporation on the (2×4)α1 and (2×4)α2 surfaces proceeds with increase of energetically favorable Ga-N bonds in the third layer via series of events such as adsorption of N-As dimer, substitution of N for As located in the third layer, and As dimer desorption to form the N-pair in the third layer. It is also found that the probability of the N-pair formation on the (2×4)α1 surface is much larger than that on the (2×4)α2 surface. This is because the strain around the N-pair on the (2×4)α1 surface is smaller than that on the (2×4)α2 surface where N-As surface dimer is formed to relax the larger strain. Furthermore, the N-pair formation probabilities P under As2 and As4 are estimated as a function of temperature including (2×4)β1. N atoms incorporated on the GaAs(001) surface can form N-pair along [110] with P ∼ 0.8 on the surface with two phase mixture of (2×4)α1 and (2×4)α2 at T = 830 K under As2 and As4.