The aim of this study is to address the essential features of experimental decaying grid-generated turbulence. Specifically, this study has focused on invariants of grid turbulence in the downstream region and Rotta model by means of direct numerical simulation based on the finite difference method. The mesh Reynolds number defined by the mesh size of the grid M, streamwise mean velocity and kinematic viscosity is 2; 500. Streamwise computational domain size is 112 M. The result shows that u²_oL⁴ is nearly constant such that u²_oL⁴ ∝ X^0:08 in the far downstream region, where X = x/M - x_o/M, x is the streamwise distance from the grid and x_o is the virtual origin. Rotta model is applicable in the region holding u²_oL⁴ = constant, and value of the Rotta constant is almost 1. The value can be well estimated by an estimation method introduced by previous experiments.