The three-dimensional (3D) numerical simulation of natural convection in a triangular cavity with the top cooling and the bottom heating is performed and compared with the previous experiment. The numerical results show that the development of natural convection in the cavity following an initially isothermal and stationary state may be classified into three stages: an initial stage, a transitional stage and a quasi-steady stage, which is consistent with the experimental result. Natural convection flows in each of the stages, particularly the 3D flow structure such as longitudinal rolls, are described. Further, the regimes of transient natural convection are discussed using a simple scaling analysis and the scaling relations obtained are validated by the present numerical results. (C) 2015 Elsevier Ltd. All rights reserved.