Unsteady natural convection heat transfer in an isosceles triangu- lar enclosure filled with Al2O3 nanoparticle is studied by using finite element method. The bottom of the isosceles triangular cav- ity is heated non-uniformly and temperature of the inclined wall is lower than that of the bottom wall. The governing parameters are Rayleigh number, solid volume fraction, aspect ratio (H/L )o f the enclosure and dimensionless-time. It is found that addition of the nanoparticle into base fluid (water) affects both heat transfer and fluid flow. Heat transfer increases with addition of nanoparticle and increasing of Rayleigh number. Conduction mode of heat transfer becomes dominant for the lower values of aspect ratio.