Using cosmological smoothed particle hydrodynamics simulations based on the concordance LCDM model, we demonstrate that the Galaxy Stellar Mass Functions (GSMFs) and UV Luminosity Functions (UV LFs) shows very steep low-mass end slopes of (a < -2) for high-z galaxies. This finding suggests that the galaxy stellar mass evolution is significantly fast at high-z. Using a high-resolution simulation with constrained realization method, we study the detail properties of high-z galaxies. The simulations show that the high-z galaxies are mostly gas rich disk galaxy that could evolve their stellar-mass very rapidly.