In this study, to improve the heat transfer performance of two-phase immersion cooling with saturated pool boiling using the “breathing phenomenon”, we evaluate the heat transfer performance of boiling surfaces composed of a lotus copper joined to a grooved surface of various groove sizes in water and FC- 72. The critical heat flux is the highest 534 W/cm 2 in the case of water using a 1.0-mm-square groove. The visualization experiments demonstrated that the breathing phenomenon is expected to be mode G, in which the vapor is mainly discharged through the grooves and the liquid is supplied from the top of the lotus copper. These experimental results verify that there is an optimal groove size to enhance the critical heat flux. In water, the maximum heat transfer coefficient is 126 kW/(m 2 •K). The maximum heat transfer performance of FC-72 is 104 W/cm 2 for the 1.5-mm-square groove. The breathing phenomenon is considered to be mode L, in which the vapor is mainly discharged from the top of the lotus copper accompanied by the liquid supply through the grooves. These saturated pool boiling experiments and the visualizations verify that the present two-phase immersion cooling can enhance both the boiling heat transfer and the critical heat flux of water and FC-72.