We study flow boiling in a rectangular minichannel of 0.5 x 4 mm(2) cross-section (hydraulic diameter: 889 gm). A two-phase flow pressure drop analysis by ranging several mass flow rates for a chosen heat flux provided to the minichannel has been performed. Two kinds of upstream conditions have been investigated to show up the confinement influence on boiling. Steady and unsteady thermo-hydraulic behaviors are reported. A stability criterion is found depending on the two controlled parameters (heat flux and mass-flow rate). The upstream condition is modified by adding a compliant buffer tank and the procedure is repeated. Unsteady flows are observed with a different intensity and with a different range of operating conditions when the flow is mainly vapor. When no compliance source is connected, the unsteady behaviors are observed in the same operating conditions. Nevertheless the characteristics of the fluctuations differ notably in amplitude and frequency. In the steady state conditions a pressure loss model is proposed based on a homogeneous assumption. A total pressure loss steady state modeling of the boiling flow is realized using a homogeneous model for the two-phase flow zone to check the steady average minichannel pressure loss. A good agreement is found with the experimental and model results. (C) 2003 Elsevier Ltd. All rights reserved.