In this paper, a series of experiments have been carried out to study different boiling instability modes of water flowing in microchannels at various heat flux and mass flux with the outlet of the channels at atmospheric pressure. Eight parallel silicon microchannels, with an identical trapezoidal cross-section having a hydraulic diameter of 186 mum and a length of 30 mm, were used in this experiment. When the wall heat flux was increased from 13.5 to 22.6 W/cm(2) and the time average mass flux of water was decreased from 14.6 to 11.2 g/cm(2) s, three kinds of unstable boiling modes were observed in the microchannels. These were (1) the liquid/two-phase alternating flow (LTAF) at low heat flux and high mass flux, (2) the continuous two-phase flow (CTF) at medium heat flux and medium mass flux, and (3) the liquid/ two-phase/vapor alternating flow (LTVAF) at high heat flux and low mass flux. Simultaneously, periodic oscillations of wall temperature. inlet and outlet water temperatures and pressures, and instantaneous mass flux were measured. Among the three unstable boiling modes, oscillation amplitudes in LTVAF were the largest with oscillations of pressures and mass flux nearly out of phase. Oscillation amplitudes in CTF were the smallest with oscillations of pressures and mass flux nearly in phase. Oscillation amplitudes in LTAF lied in between LTVAF and CTF with oscillations of pressures and mass flux nearly out of phase. Also, the oscillation period (including two-phase period, liquid period and vapor period) depends greatly on the amounts of heat flux and mass flux. Bubbly flow and some peculiar two-phase flow patterns were observed in the microchannels during two-phase flow periods. (C) 2004 Elsevier Ltd. All rights reserved.