During the heat transfer of phase change materials in a closed cavity, natural convection and contact melting play important roles in heat transfer enhancement. An improved Nusselt boundary layer model is proposed to describe the contact melting process of phase change material in a rectangular cavity at different tilt angles. A 2-dimensional visualization experimental set for contact melting process of phase change material is put up. The melting process of n-octadecane in a rectangular cavity at 7 different tilt angles (0 degrees, 15 degrees, 30 degrees, 45 degrees, 60 degrees, 75 degrees, 90 degrees) is recorded by digital camera. The liquid fraction, dimensionless height and the total melting time of the phase change materials in the tilted rectangular cavity are obtained through digital image processing mothed. The dimensionless height of solid phase change materials and liquid fraction are consistent with the theoretical solution. And the shortest total melting time is when the tilt angle is 60 degrees. (C) 2017 Elsevier Ltd. All rights reserved.