Abstarct: Double diffusion natural convection in a square cavity in the presence of a hot square obstacle is simulated using the lattice Boltzmann method (LBM). Lattice Boltzmann equations for flow, temperature and concentration are presented and by comparing different hydrodynamic, thermal and concentration boundary conditions, it was found out that in a problem with straight walls, it is convenient to use first order accuracy boundary conditions and there is no need to use second order accuracy solvers in LBM. Then double diffusive natural convection in square cavity is studied via LBM while horizontal walls are adiabatic and impermeable and vertical walls of the cavity have constant temperature and concentration. A hot obstacle is in the center of the cavity which all of its walls have temperature and concentration of unity. The Prandtl number is 0.71. Viscosity is set to 0.02. The results are presented for Rayleigh numbers 104,105 and 106, Lewis numbers 0.1, 2 and 10 and aspect ratio A (obstacle height/cavity height) of 0.2, 0.4 and 0.6 for a range of buoyancy number N=0 to -4. In this study the effect of aiding flow (N>0) is not the subject of interest. The results show that when |N|<1, as buoyancy ratio increases, the Nusselt number and Sherwood number decrease with |N| and when |N|>1, these numbers rise with |N|, therefore a minimum value is observed in the Nusselt number per buoyancy ratio diagram. The Nusselt and Sherwood numbers increase with Rayleigh number and aspect ratio in a certain buoyancy ratio which this indicates the growth in heat and mass transfer rate. The flow patterns show that with increase of Rayleigh number, the multi-cell flow will form in the enclosure. As a result of rise in the effect of temperature gradient in respect to that of concentration gradient, parts of the flow will form vortices which their direction is opposite of that of main flow. These vortices will vanish as N increases. It is observed that as Lewis number increases, higher buoyancy ratio is required for concentration effect to overcome the effect of thermal diffusion Also Sherwood number increases with Lewis number.