A comprehensive model is developed to study the heating, melting and evaporation of powder particles in plasma flames. The well-established LAVA code, a CFD software specially designed for plasma flame simulation, is used to predict the plasma gas field under given power conditions, and provide inputs to the particle model. The particle is assumed to be spherical and one-dimensional heat conduction equation with phase change within the particle is solved numerically using an appropriate coordinate transformation and finite difference method. Melting and vaporization interfaces are tracked and the particle vaporization is accounted for by the mass diffusion of vapor through the boundary layer around the particle. The model also takes into account of both the convective and radiative heat transfer between the particle and the plasma. The effect of mass transfer on convective heat transfer is also included. Calculations have been carried out for a single particle injected into an Ar-H2 plasma jet from outside the DC-plasma gun. Zirconia and nickel are selected as solid particles because of their widespread industrial applications as well as the apparent difference in their thermal properties. Numerical results show strong non-isothermal effect of heating, especially for materials with low thermal conductivity, such as Zirconia. The model also predicts strong evaporation of the material at high temperatures. A parametric study has been performed to examine the physical mechanisms of particle melting and evaporation in plasma and the effect of vaporization on convective heat transfer.