In this study, experimental results of the micro-pump and volume displacement methods were compared to elucidate the root-cause of hysteresis in capillary pressure measurements. It was shown that hysteresis between the imbibition curve and drainage curve was apparent in the capillary curve of the micro-pump system. The measured capillary pressure curves by the micro-pump technique were observed to be highly dependent on the pump rate. Results of the volume displacement method showed that the hydrophobic layer affected both imbibition and drainage processes. Results showed higher pressure was needed to introduce water into the sample and hysteresis was observed only when the hydrophobic layer was used.