Cavitation refers to a nucleation phenomenon that occurs at room temperature when the liquid pressure is below the corresponding saturation vapor pressure. Although water confined by a nanochannel and a liquid-air meniscus is under negative pressure, i.e. much smaller than the corresponding satraton vapor pressure, cavitation has not been observed in any nanochannels. In this work, we report our observation and studies of cavitations in nanochannels for the first time. 1-D confined nanochannels for this work were fabricated based on a sacrificial-layer-etching scheme. The unique cavitation phenomenon occurred when water started evaporation at the nanochannel entrances. Instead of meniscus recession, a bubble was present inside the nanochannel and two meniscii were pinned at the entrances. This bubble started growing along both directions until it totally occupied the whole channel. We found that the bubble grows linearly with time and the bubble growth rate decreases with the increasing channel height. A theoretical model was developed to study this dynamic process. It is found that the bubble growth rate is determined by the evaporation rate at the entrance. Since the total evaporation flux is a constant, the predicted bubble growth rate is reversely proportional to the channel height, quantitatively consistent with the experimental results. Since most current studies for caviation are theoretical studies, our studies provide a new experimental approach to study these phenomena in artificial transparent nanochannel devices.