High-speed visualization is used to investigate local flow boiling regime development of R134a in parallel microchannels with a hydraulic diameter of 1.1 mm and a high channel wall to width ratio. The effect of heat and mass flux along with the microchannel geometry on the flow boiling regimes is studied. The ranges of parameters are: mass flux from 135.7 to 339.3 kg/m²-s, heat flux from 0 to 33.7 W/cm², inlet temperature of 10 °C, and vapor quality from subcooled to dryout. The thick channel walls create low-pressure zones at the channel inlet due to an abrupt reduction in flow area. The combination of low-pressure zones and strong conduction effects in the channel walls create an ideal condition for bubble nucleation in the microchannel. At low heat fluxes, bubbly flow prevails at the channel inlet which quickly transitions to slug flow as the bubbles coalesce. As the heat flux increases, the flow transitions to a cavitation- and conduction- driven two-phase flow regime called jet flow. The dynamics and mechanism of the jet flow are explained using high-speed visualization. An early transition to wavy-annular flow at the channel inlet is observed which enhances the heat transfer for high and mass heat fluxes.