In this paper, free convection heat transfer of Al2O3/water nanofluid within an enclosed cavity is studied by adopting the Lattice Boltzmann Model. The left and right side walls of the cavity have a complex-wavy surface and the left wall is heated by a sinusoidal temperature distribution higher than the right cold wall. The top and the bottom horizontal walls are smooth and insulated against heat and mass. In this study, the variation of density is slight thus by using the Boussinesq approximation would be influencing the Hydrodynamics field of the thermal field. The density and energy distribution are both solved by D2Q9 model. The influence of pertinent parameters such as solid volume fraction of nanoparticles, Rayleigh numbers, complex-wavy-wall geometry parameters, phase deviation and amplitude of the sinusoidal temperature function on flow and heat transfer fields are investigated. Results show for Rayleigh numbers in the range of Ra=103 -105, with increasing volume fraction of nanoparticles, Nusselt number increases. In addition, it is shown that for a fixed Rayleigh number, the heat transfer performance depends on tuning the wavy-surface geometry parameters. The greatest effects of nanoparticles are observed for different values of the phase deviation with increasing of Rayleigh number. This study can, provide a useful insight for enhancing the convection heat transfer performance within enclosed cavities with complex-wavy-wall surfaces and sinusoidal temperature distribution.