Abstract: For simplicity in theoretical analysis and quantitative calculation, 3D model is often simplified to 1D model. 3D and 1D models for 2 samples, one is a CFRP panel and another is an aluminum one, are built and calculated by FEM method. The results showed that influence of lateral heat diffusion was more serious in materials with greater thermal diffusivity. An experiment was carried out on a GFRP/NOMEX honeycomb panel. It was seen that lateral heat diffusion became obvious in materials with low thermal diffusivity as time goes by. In conclusion, error should be estimated before using 1D model instead of 3D model for simulation and analysis in infrared thermographic NDT.