Knowledge of transient critical heat flux (CHF) for subcooled boiling of water flow is important for the design of cooling devices such as fusion reactor divertor. Heating occurs rapidly in the divertor and the heat flux is estimated to reach 10 MW/m² because of plasma disruption. To cope with such a heat flux under the transient condition, the use of a small inner diameter tube (< 3 mm diameter) is recommended, since the use of a smaller inner diameter tube results in a higher CHF. The transient CHF of subcooled boiling of water was measured for a 1.0 mm inner diameter tube, which was heated exponentially, with inlet subcoolings ranged from 82 to 145 K and a flow velocity of 10 m/s. The heat generation rate of the tube was exponentially increased according to the function Q₀exp(t/τ). E-folding time, τ, ranged from 182 ms to 30.5 s. It was found that the transient CHF increased as the e-folding time decreased and that CHF of the small-diameter tube was higher than that of conventional channels. This indicated that CHF is affected by the e-folding time under transient conditions.