The buoyancy-driven heat transfer enhancement and fluid flow of nanofluids inside inclined enclosures in the presence of heat generation/absorption effect are investigated in this paper. The bottom and top walls of the enclosure are thermally insulated, while both side walls are considered to have sinusoidal distributions of thermal boundary conditions. Two cases are considered; an enclosure containing a cold obstacle and the other one contains a cold circular cylinder. The governing equations are converted to dimensionless forms and then solved using Galerkin finite element method. Effects of the key parameters, namely, Rayleigh number, nanoparticle volume fraction, cavity inclination angle, heat generation/absorption parameter, amplitude parameter, phase angle, size and position of the inner shapes on the contours of streamlines and isotherms as well as average Nusselt number are examined. It is found that an enclosure with a square obstacle enhances the heat transfer rate with a higher rate comparing with the circular cylinder case. Also, Considering an inclined cavity gives rate of heat transfer greater than the horizontal/vertical cavity. In addition, regardless the inner shape, the average Nusselt number is an increasing function of nanoparticle volume fraction, amplitude parameter and phase angle. (C) 2018 Elsevier Ltd. All rights reserved.