To study the influence of porosity on photothermal radiometry (PTR) signals of carbon fiber reinforced polymers (CFRPs), a four- layered PTR model is developed and PTR measurements are performed. Three laser beams with different diameters are used to measure 9 CFRP samples with varied porosity levels. The measured amplitudes and phases of the PTR signals are normalized. The result shows that the phase of PTR signal increases with decreased beam size if the laser power are kept constant. For a beam size of 0.2 mm, the PTR phase increases when the CFRP sample porosity decreases.