A new method is presented to measure capillarity-induced negative pressure. Negative pressures of several bars have been measured for five different liquids (ethanol, acetone, cyclohexane, aniline, and water) over a range of surface tension. Capillary negative pressure wits measured in 79 +/- 3 nm silica nanochannels on the basis of the determination of the critical channel width for elastocapillary collapse of the flexible plate covering the channels. The results are consistent with the Young-Laplace equation.