Elongated bubble flow is a unique flow pattern that occurs during flow boiling in a microchannel. The incepted bubbles quickly grow to the channel size and form long elongated bubbles due to the confinement of the microchannel. The flow regimes and heat transfer coefficients in the microchannel are closely related to the elongated bubble behavior. Numerical and experimental investigations were performed to study the behavior of elongated bubbles and their effect on flow boiling. Elongated bubble growth in a $100-{mu}m-diameter$ microchannel was calculated by using a two-zone model, while flow visualization of elongated bubble behavior was obtained with a high-speed CCD camera. Comparisons between the calculated results and experimental observations showed that the growth of the elongated bubble was governed by the evaporation of the thin liquid film that formed between the vapor core and the heated channel wall.