The large eddy simulation method has been used to simulate the diffusion of H₂SO₄ vapor in the parallel twin jets. The distributions of number concentration and size of nanoparticles produced by nucleation and coagulation in sulfuric acid/water system are given. The functions of the sulfur content, relative humidity and jet Reynolds number are evaluated according to the distributions of number concentration and size of nanoparticles. The results show that the nucleation in sulfuric acid/water system produces large number of nanoparticles , and gas-to-nanoparticle conversion mostly takes place in the middle and interface of the twin jets. The coagulation process of particles reduces the number concentration, while increases the mean particle size. For the case with higher sulfur content, more number and smaller size nanoparticles are produced by nucleation and coagulation. There is also a larger number of nanoparticles for the cases with higher relative humidity and jet Reynolds number.