The homogeneous charge compression ignition (HCCI) combustion process is capable of providing both high ‘diesel-like’ efficiencies and very low NOx and particulate emissions. However, among several technical challenges, controlling the combustion phasing, particularly during transients is a major issue, which must be resolved to exploit its commercial applications. This study is focused on the experimental investigations of behavior of combustion timing and other combustion parameters during startup and load transients. The study is conducted on a gasoline fuelled HCCI engine by varying intake air temperature and air-fuel ratio at different engine speeds. Port fuel injection technique is used for preparing homogeneous mixture of gasoline and air. For fueling startup transient test, fuel injection was turned off, and the engine was motored for several minutes until the fire-deck, intake and exhaust temperatures stabilized. The fuel injection was then turned back on and the in-cylinder pressure was recorded for combustion analysis using piezoelectric pressure transducer. Each test was repeated 5 times and the ensemble average of these transients was analyzed for different combustion parameters. Results show that combustion phasing shifts pattern during a fueling-startup transient depending on the intake air temperature and air-fuel ratio.To determine the effect of transients on the HCCI combustion process, two types of transient fueling tests (step and pulse) were performed. The first test was a simple step starting at the lower load condition, and then stepping fueling to a higher load. Second test is a pulse-in-fueling, where fueling for the lower load condition is stepped to the higher load condition for several hundred cycles, and then stepped back to the minimum load case. The step transient was used to evaluate relatively short dynamics of the change in fueling, as well as any relatively long effects such as cylinder wall heating. The pulse transient was used to show the effect of both increasing and decreasing fueling on the engine response. In pulse transient test, pulse widths of 200, 300, 400 and 500 cycles were tested and results are analyzed.