This work is devoted to the study of existence of positive solutions and hydrodynamic limit of the steady Boltzmann equation with in-flow boundary condition. The proof is based on a L6– L∞ framework developed by [10] and a refined positivity-preserving scheme in deriving positivity of solutions with in-flow boundary condition and external force. The incompressible Navier–Stokes–Fourier limit with Dirichlet boundary condition is justified for in-flow boundary data as small perturbation of a global Maxwellian.