Alkylation of benzene with ethylene to ethylbenzene over a beta-zeolite catalyst is carried out in the five different near critical regions. With the application of X-ray diffraction (XRD) and scanning electron microscope (SEM), structures of the catalysts used under the five different near critical conditions are compared with those of the fresh catalyst. The results show that the high pressure operating conditions, including the supercritical fluid operation, may be harmful to the catalyst structure, and the operation in the vapor-liquid state near the critical point may cause heavy destruction to the catalyst. The coking mechanism of the catalysts is suggested based on element material analysis (EMA) and electron paramagnetic resonance (EPR) characterization. Supercritical and other near critical operations are compared.