In reciprocating machinery such as IC engines, where the amount of frictional power involved during the motion of the piston inside the cylinder is very high, proper lubrication of the cylinder liner is critical. A lack of sufficient lubrication inside the cylinder can lead to a wide range of problems, including power loss, excessive fuel consumption and, in more serious cases, scuffing and damage to the liner and rings. Thus, monitoring of the lubricant oil supply to the cylinder is essential in achieving better efficiency for the engine in terms of power, fuel consumption, etc. This paper deals with the use of acoustic emission signals to evaluate the effectiveness of the lubrication inside the cylinder. Tests were conducted on two different kinds of engine to study the relationship between the lubricant oil level and the frequency content of acoustic emission (AE) signals. Furthermore, the changes in the AE signals were studied quantitatively to prove the effectiveness of AE as a tool for monitoring lubricant oil level.