Use of a hydraulic lash adjuster (HLA) in the valve train has some problems as well as several advantages. One of the problems is less stiffness of valve train, which may be further reduced when aeration of the oil in the oil circuit occurs. The much lower stiffness will lead to malfunction, noise and damage with the valve train. So the aeration of oil in HLA high pressure chamber must be considered in modeling and simulation of the valve train. Since air dissolved capacity in the oil varies with pressure, aeration (undissolved air content) in the high pressure chamber is different from that in the oil circuit. So far, only the air content in the oil circuit has been measured, and few reports have been found on determining the aeration in HLA of an operating valve train. Based on knowledge that HLA collapse is caused by compressibility and leakage of oil, a method of determining the aeration of the oil in the high pressure chamber is introduced. By subtracting the reduction in collapse of the HLA caused by oil leakage from the total measured collapse, the part caused only by oil compressibility is left. According to the remaining reduced lift and force applied on the HLA by the valve assembly, the stiffness and bulk modulus of the oil/air mixture may be calculated. Then the volumetric air fraction may be determined from the bulk modulus of pure air and pure oil.