Dissolving CO2 into the ocean by the pumping of liquid CO2 is a strategy to offset the buildup of atmospheric CO2 However, under the high-pressure, low-temperature conditions in the deep ocean a CO2 hydrate film quickly isolates the liquid CO2 from seawater thus: suppressing dissolution. The thickness of the hydrate film is one of the most important parameters for understanding the mass transport mechanism through the hydrate film during the liquid CO2 dissolution. We suggest a new method for measuring the thickness of the hydrate film formed at the interface between the liquid CO2 and flowing water. Our method assumes that a unique thickness of hydrate film is kept through hydrate dissolution and growth at the water-, and Liquid CO2-hydrate interfaces, respectively. When growth stops, the net dissolution allows estimation of the original hydrate thickness. Under the conditions of our experiment, the CO2 hydrate film thickness is about 0.4 mu m.