A process of hydrogen peroxide generation in an underwater capillary discharge with a different geometry of the current conducting channel has been investigated. Measurement of H2O2 has been carried out by means of spectrophotometry with a reaction of ammonium metavanadate in an acid medium. It is found that an efficiency of hydrogen peroxide formation depends on input power dissipated in the discharge and on length of the capillary. It is shown that maximal rate of the hydrogen peroxide formation is 0.11 mmol/l⋅s with the equal capillary length of to the in⋅y 1 mm. The efficiency of H2O2 formation in 1 mm capillary decreases to 4.1 g/kW⋅h after a long treatine time by compare with 5 mm capillary (efficiency is 7,8 g/kW*h). A maximum yield of H2O2 in the capillary discharge is higher than in the traditional diaphragm one or in a glow discharge with electrolyte cathode. The maximum hydrogen peroxide yield amounts to 3.48 mol/F.