Blood cell counting and classification is one of the most commonly ordered clinical laboratory tests. It is a useful screening and diagnostic test often done as part of a routine physical examination. Blood cells can be divided into three types: red blood cells, white blood cells and blood platelets which are different in sizes, properties and functions.Coulter Counters are electrofluidic devices commonly used for measurement of microscopic particles such as in blood samples analysis. The Coulter Counter helped revolutionize the complete blood count (CBC), the standard initial blood test for a majority of medical practices. Blood quality can be analyzed rapidly, objectively, as well as quantitatively. In the Coulter Counter device, the resistance will change when the particles flow through the microchannel. By measuring the change of resistance, the sizes of particles can be obtained through calculation. Blood cells suspended in a conductive liquid act as electrical insulators. The basic assumption underlying the Coulter principle is that the voltage pulse generated when a particle passes through the aperture is directly proportional to the particle volume.In this research an electrical circuit was designed using an instrumentation amplifier and a low-pass filter to measure the change in resistance of the pore. The fundamental op-amp circuit used as the instrumentation amplifier amplifies the difference between the input signals which represent the difference between the voltages of the resistance of the liquid volume between the electrodes and the particle, compared with the voltage of the resistance without the particle. As the frequency of the pulse signal caused by the flowing particle was 0.7Hz to 8Hz (the number of particles that flows through the pore per second is about 0.7~8/per second). At high frequency there would be only noise component. Thus, a low-pass filter was used to filter the high frequency noise.This circuit properties was simulated using Multisim7 by Electronics Workbench program and the results of this simulation was compared to the experimental results which implied that this circuit was reliable to measure particles from 0.1μm-15μm (since the maximum blood cell size is 12.5μmand fit the design of the instrumentation amplifier and the low-pass filter with the bandwidth of 0.5KHz.