High-speed photography was applied to visualize the flow pattern evolution of flow boiling of liquid nitrogen in an upward quartz microtube coated with a layer of transparent indium tin oxide film as the heater. The inner diameter of the employed tube was about 1.33 mm. The characteristic of nucleation site activation, which was the beginning of the flow pattern evolution, was studied visually. In the case of low heat flux and small mass flux, only a single nucleation site was activated. The departed bubbles grew to the size of the inner diameter of the tube soon and the transition from the bubbly flow to slug and annular flow occurred earlier compared to the conventional normal-sized tubes. In the case of high mass flux and high subcooling, multiple nucleation sites were activated. The interaction between adjacent nucleation sites was considered. Slug and annular flow were found to suppress the downstream nucleation site. Bubble coalescence was one of the key factors for the flow pattern evolution, and it was found that the bubble coalescence would bring about great disturbance to the adjacent nucleation site. The characteristics of bubble condensation in the subcooled liquid nitrogen were also demonstrated. Flow pattern evolution beyond the boiling crisis was also investigated. Post-dryout regimes such as inverted bubbly, inverted slug, and inverted annular flow were observed in the microtube. Flow reversal and liquid entrainment, which were relevant to flow instability in the flow pattern evolution, were demonstrated clearly.