Provided are a biosensor and a method of driving the same. The biosensor includes a transistor including a substrate including a source, a drain, and a channel formed between the source and the drain, a gate insulating layer formed on the channel, and a source electrode and a drain electrode respectively connected with the source and the drain, a fluid line for covering the transistor to have an inner space together with the transistor and in which a sample solution including target molecules flows, a reference electrode formed on an inner wall of the fluid line, and a probe molecule layer attached on the reference electrode and reacting with the target molecules. Accordingly, the reference electrode is formed on the inner wall of the fluid line, enabling miniaturization of the bio device. Also, the probe molecules are formed on the reference electrode to measure a change in threshold voltage according to a change in electric potential between the reference electrode and the gate insulating layer, such that the sensitivity and reaction rate can be remarkably improved.