Methane storage data of clathrate hydrates formed in ternary system, methane-tetra-n-butyl ammonium bromide-water (CH4-TBAB-H2O), was measured by a magnetic suspension weight adsorption instrument at the temperature of 273.64 ~ 278.23 K and the pressure of 2.01 ~ 7.07 MPa, for two concentration (20.8 wt % and 43.3 wt %) of TBAB solutions. The hydrates component was qualitatively analyzed. The results showed that the structure of CH4-TBAB semi-clathrate hydrates was deformed at high pressure possibly due to methane molecules replacing TBAB molecules and forming structure I true methane hydrates for the 20.8 wt % TBAB solutions at 273.64 K. For 43.3 wt % TBAB solutions, at the same experimental temperature and pressure, the maximum mass fraction of methane in CH4-TBAB-H2O clathrate hydrates was only 1.64 wt % which was less than the ideal maximum value (3.08 wt %). It showed that for the 20.8 wt % TBAB solutions, TBAB molecules were not replaced by methane molecules but both CH4-TBAB semi-clathrate hydrates and structure I true methane hydrates were formed simultaneously. For 43.3 wt % TBAB solutions, the reason of low methane storage capacity may be that the pressure was not high enough.