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The method to synthesize single-walled carbon nanohorns ͑SWCNHs͒ using gas-injected arc in water ͑GI-AIW͒ has been experimentally studied. GI-AIW is known as one of the cost-effective methods to obtain SWCNHs. It was revealed that the yield of SWCNHs significantly decreases with the increase in water temperature although the purity of SWCNHs is not dependent on the temperature change. Then the model of relevant reactions in the GI-AIW system was proposed by accounting the emission of carbon vapor, formation of SWCNHs, and diffusion of water vapor in three zones inside the cathode hole ͑arc plasma zone, quenching zone, and downstream zone͒. The side reaction between H 2 O and C produces H 2 gas and consumes a certain amount of carbon vapor, resulting in the hindered SWCNH formation. Moreover the observation of the optical spectra emitting from the arc plasma zone strongly supported that the H 2 generating reaction does not occur at arc plasma zone since N 2 flow can purge H 2 O out. The model proposed in this study can precisely explain the correlation between H 2 gas production and water temperature.