A method for synthesizing ultrafine zeolite A crystals with narrow particle size distribution (PSD) using a watera??liquid paraffin two-phase segmented microfluidic device assembled by coaxial dual tubes is reported. The influence of the sizes of the outer tubes, the aging of the synthesis solution, and the reaction temperature was examined. As-synthesized zeolite crystals were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, dynamic light scattering, energy dispersive X-ray spectroscopy, scanning electron microscopy, and field-emission scanning electron microscope. Zeolite A crystals with mean particle sizes (MS) of 408 and 262 nm and PSDs of 150a??620 and 100a??420 nm were prepared in the segmented microfluidic reactor (inner tube i.d. of 0.6 mm, outer tube i.d. of 1 mm) at 80 A?C for 20 min using the fresh synthesis solution and for 5 min using the aged synthesis solution, respectively. The MS and PSD of zeolite A can be controlled by adjusting the temperature and the sizes of the outer tube. The crystallization performance of zeolite A in the segmented microfluidic reactor was compared with those in the conventional and microwave-heated batch reactor and the single-phase microfluidic reactor at 90 A?C. It was found that the rate of crystallization in the segmented microfluidic reactor was a bit slower than that in the microwave-heated batch reactor, but was much higher than those in the conventional batch reactor and single-phase microfluidic reactor. The MS and PSD of the resulting zeolite A synthesized in the segmented microfluidic device are smaller and narrower than those produced in the conventional batch reactor, single-phase microfluidic reactor, and microwave-heated reactor.