Abstract <p>The energy gap, formed between the lowest unoccupied molecular orbital and highest occupied molecular orbital, serves as a critical role in determining some key performance of organic-based device. This review makes an a comprehensive clarification of different gaps, and the corresponding scenario in which they will be applied. Firstly, the electrochemical gap is measured by cyclic voltammetry method which determines that this value is extremely easy to obtain. However, the reproducibility of electrochemical gap is not very well, because it is easily affected by the testing environment. Secondly, the optical gap is determined by UV-Vis absorption spectroscopy, and it is usally apllied in opto-electrical devices. Due to the photo generated excitons during the measurement process, the value is always larger than LUMO-HOMO. Thirdly, the electrical gap is detected by ballistic electron emission spectroscopy and hot electron transistor. Owing to the use of real electronic device structures in this test, its value is closest to LUMO-HOMO. Finally, the interfacial gap is obtained by combination of ultraviolet electron spectroscopy and the inverse photoemission electron spectroscopy. This value illustrates an interfacial attribute of organic semiconductor which is suitable for the mechanism study of thin film devices.</p>