We report a new sensing technique of proteins using competitive proteins' displacement reaction on a surface, namely Vroman effect. A target protein displaces a pre-adsorbed weak-affinity protein; however a pre-adsorbed strong affinity protein is not displaced by the target protein. In a microfluidic device, we engineer two gold surfaces covered by two known proteins. The sensor allows selective protein detection by being displaced by a target protein on only one of the surfaces. The SPR (Surface Plasmon Resonance) sensorgrams show that three different human serum proteins, immunoglobulin G (IgG), tyroglobulin (Tg) and fibrinogen (Fib) have different adsorption strengths to the surface and the competitive adsorption of individuals controls the exchange sequence. Based on the exchange reaction, we demonstrate that the sensor has a high selectivity for Tg. Immunosensor techniques have become the dominant test methods in diagnostics, therapeutics and protein research, partially due to the highly selective molecular recognition of antibody and antigen. However, they often suffer from cross-reactivity, non-specific adsorption and lack of antibody diversity. Besides these limitations, integrating antibodies on to a transducer is a time-consuming and labor intensive process and often become the bottle neck of high yield sensors. To date, few alternative platforms for the protein detection have been active in biosensor communities. Here, we report a fundamentally different protein detection method that relies on the competitive nature of protein adsorption onto a surface, namely the Vroman effect. The Vroman effect is governed by thermodynamics as it is more thermodynamically stable in nature. By using the technique, we obviate the need to rely on antibodies and their attachment to transducers. Our approach is that one can engineer two surface pre-absorbed by two known proteins; one is a little smaller and the other is a little bigger molecular weight proteins than the target protein. Then, the pair of the surfaces becomes a highly-selective protein sensor since one is displaced an the other is not displaced by the target protein. In its first implementation, we demonstrate that three human serum proteins, IgG, Tg, and Fib, have different adsorption strengths onto a hydrophobic gold surface. The different strengths induce an exchange reaction among them. The displacement strength is ranked in the following order; Fib (340 kDa) > Tg (660 kDa) > IgG (150 kDa). In other words, fibrinogen can displace all other proteins while Tg only can displace IgG. Based on the results, we can identify specific target proteins without using the conventional immunosensor technique. Our results show how to detect Tg using a pair of surfaces pre-adsorbed by two known-size proteins; IgG in channel 1 and Fib in channel 2. Tg displaces IgG in channel 1 but just flows through the fibrinogen-covered surface in channel 2 without any exchange reaction. The differential measurement of the SPR angle change from channel 1 and 2 allows the detection of Tg and the angle change also indicates how many thyroglobulins replace IgG.