A great deal of effort has been directed towards Gasoline HCCI engines, which have the potential of providing better fuel economy and emission characteristics than conventional SI engines. For stable HCCI engine operation, cycle-by-cycle based closed-loop control is needed. Such a control scheme requires an accurate and reliable sensor to monitor the combustion and provide a feedback signal. At present, the general method used to measure the combustion parameters is to monitor in-cylinder pressure with a cylinder pressure sensor. However, using in-cylinder pressure transducers is not feasible for use in mass production of HCCI engines. A good substitute to get information about combustion is the knock sensor, which is already equipped on engines on a large scale. In this paper, the knock signal from an HCCI engine equipped with 4VVAS is analyzed in detail to find the relationship between the combustion parameters and the knock sensor signal. A Fast Fourier Transform is used to extract the main characteristics from the knock sensor signal. The use of a number of different parameters from this signal to represent standard combustion parameters is investigated, and the usefulness of the different knock characteristic signal is compared. The results show that there is strong relationship between the knock sensor signal and the combustion parameters such as CA10, maximum pressure and maximum pressure rise rate. The correlation coefficient between CA10 and the start phase of the knock signal has a high value of 0.978. The knock sensor signal provides a feasible and cost effective way to get combustion parameters of the HCCI engine.