Objective: To establish the fluctuating curve of intra-articular pressure in the temporomandibular joint (TMJ) with anterior disc displacement without reduction and to discuss the mechanism of formation. Methods: Twenty-two affected sides were identified in 16 patients. A No. 8 syringe needle was used to pierce the upper compartment of the TMJ. The pre-arthroscopy intra-articular pressure was measured by the pressure transducer via the flexural rigid tubing in open and closed positions. The curve and mean value were documented. Results: The pressure values were classified into 3 types: high pressure, reverse pressure and low pressure. The first type manifested a general increase and wide amplitude in pressure averaging 7.11 ± 17.87 mmHg in open position and 9.65 ± 7.03 mmHg in closed position. Contradictory to the first type, the second type showed extraordinary variation in pressure, 7.48 ± 8.49 mmHg in open position and -3.91 ± 6.25 mmHg in closed position. The patients of the last type had significantly low negative intra-articular pressure, which averaged -114.49 ± 91.61 mmHg in open position and -61.24 ± 24.03 mmHg in closed position. Conclusion: The TMJs with anterior disc displacement without reduction can be classified into 3 types: high pressure, reverse pressure, and low pressure.