Hemodynamics plays an important role in the formation of blood clots, for which changes in hydrodynamic stresses and transport phenomena can initiate or inhibit the clotting process. Fibrin, which is converted from fibrinogen in blood plasma, plays a dominant role in structural mechanics of a clot. Clot analogs are conventionally fabricated in a static in vitro environment whereas clot formation in vivo occurs in the presence of dynamic blood flow. In this paper we demonstrate an ability to produce clot analogs at the boundary between active co-flow fluid streams. The time evolution of clot formation in microchannel flow was investigated using fluorescence imaging of fibrin clots at one-minute intervals. Time-tracking of skewness and kurtosis of fluorescence intensity data was conducted to monitor shape and density distribution changes in the clot. Soft lithography and casting techniques were used to fabricate a polydimethylsiloxane (PDMS) microfluidic device which consisted of a Y-shaped microchannel 300 μm wide × 12 μm deep × 10 mm long with two inlets and a single outlet. The first inlet introduced fresh frozen plasma (FFP), which contains fibrinogen and plasma proteins. The second inlet introduced thrombin, which initiated the conversion of fibrinogen to fibrin. Clot analogs were formed at the interface between these two parallel streams. Flow was driven by withdrawal of a syringe pump at flow rates of 50 nL/min and 100 nL/min. Clots that are formed in such an engineered device provide opportunities to recapitulate the flow rates and concentrations of reagents, to mimic in vivo scenarios in which clot density and composition gradients depend on flow conditions.