Experiments were performed to investigate thermodynamic states in micro pulsating heat pipes (MPHPs) in a horizontal orientation. The rectangular microchannel in the MPHPs with 10 and 15 turns was engraved on the silicon wafer with a thickness of 1 mm to form a closed-loop. The width and height of the channels were 1 mm and 0.5 mm, respectively. The pressure variations of vapor plugs were measured using the pressure transducers in the adiabatic section. Simultaneously, the temperature variations of vapor plugs and liquid slugs were obtained by micro-thermocouples. For the flow visualization, a Pyrex glass with a thickness of 0.7 mm was used as a cover. The MPHP with 10 turns exhibits a large-amplitude oscillation followed by a long-term stopover phenomenon repeatedly, while the MPHP with 15 turns shows a relatively stable oscillation with the large amplitude. In the stopover region, vapor plugs occupy the channels in the evaporator section, while liquid slugs occupy the channels in the condenser section. It results in a dryout in the evaporator section, and the vapor plugs are superheated. In addition, there exists very large temperature gradient in vapor plugs. On the other hand, in a stable operation region with the large amplitude, the two-phase mixture having a complex flow behavior oscillates from the evaporator to the condenser. During the large-amplitude oscillation, it is identified that vapor plugs are alternately saturated and locally superheated.