Experiments were conducted to study the temperature field,flow field,3-D characteristics and self-sustained oscillation of the natural air convection in a 3-D shallow cavity which was heated from underneath. The experiments were conducted by the methods of laser holographic interferometry photography and smoke visualization. To ensure the temperature of both plates to be constant and the accuracy of laser interferometer,the instrument was calibrated and error has was analyzed. The results showed that the flow field was stable at lower Rayleigh numbers. When the Rayleigh number increased,the flow field became instable and the isotherms distorted. The rolls merged at Ra=12500 and formed along both axes when Ra was over 18500. The air rose from the middle and descended in the circumference while the flow field and heat transfer converted to 3-D characteristic from 2-D characteristic. When the Rayleigh number increased,the flow field became more instable. The rolls became irregular and time dependent when Ra≥Rac(=30500) ,which is nonlinear. When lateral walls were heated or cooled,the rolls merged along the long axis and two rolls formed along the short axis. Three rolls formed occasionally in the process. Experiments were conducted to study the temperature field, flow field, 3-D characteristics and self-sustained oscillation of the natural air convection in a 3-D shallow cavity which was heated from underneath. The experiments were conducted by the methods of laser holographic interferometry To ensure the temperature of both plates to be constant and the accuracy of laser interferometer, the instrument was calibrated and error has was analyzed. The results was that the flow field was stable at lower Rayleigh numbers. When the Rayleigh number increased, the flow field became instable and the isotherms distorted. The rolls merged at Ra = 12500 and formed along both axes when Ra was over 18500. The air rose from the middle and descended in the circumference while the flow field and heat transfer converted to 3-D characteristic from 2-D characteristic. When the Rayleigh number increased, the flow field became more instable. The rolls became irregular and tim e is when when Ra≥Rac (= 30500), which is nonlinear. When lateral walls were heated or cooled, the rolls merged along the long axis and two rolls formed along the short axis.