A quasi common path phase-shifting interferometer for convection measurement was developed in this paper. This interferometer has a large measurement area of approximately 100 mm in diameter to visualize the spatial structure of natural convection. It also has a quasi common path to reduce the effects of air disturbance and vibration. In addition, a phase-shifting technique is introduced to improve the interferometer's phase resolution and spatial resolution. Natural convection fields around a vertical flat plate with Rayleigh numbers from 1.9 x 10(6) to 3.0 x 10(6) were measured and the temperature profiles were estimated by the interferometer. The obtained temperature profiles were compared with the analytical solutions. Moreover, the three-dimensional effect of a thermal boundary layer was investigated. To validate the interferometer's capability, the experimental results were compared with the calculated results in terms of changes in the optical path length of the test beam. It was confirmed that the quasi common path phase-shifting interferometer can measure the optical path length change accurately at high phase and spatial resolutions. (C) 2012 Elsevier Ltd. All rights reserved.