New regulations of the International Maritime Organization (IMO), introducing drastic reductions in fuel sulfur content, allow 0.1% sulfur in fuels used in emission control areas (ECA), starting from 2015. Together with the worldwide situation of decreasing fuel resources, the introduction of alternative fuels complying with future regulations of marine diesel fuel displays an important research these days. Light Cycle Oil (LCO) also referred to as 'Cracked gas oil', a sub-product from the FCC process in refining, has the potential to be used as an alternative for current marine fuels. LCO shows a high content of aromatic hydrocarbons, mainly composed of one and/or two ring aromatics. In general, aromatics are chemically less reactive compared to paraffinic compounds and therefore it can lead to a strong deterioration of the ignition and combustion characteristics. Especially for medium and high-speed engines, the deteriorated combustion characteristics such as long ignition delay, long after-burning and long flame-length could result in the trouble for the piston ring and cylinder liner causing the dry-out of the lubricating oil film. Pilot injection is one of the techniques to reduce the difficulty in the engine operation with low ignitability fuel. Experimental results show that the pilot injection enables the engine to operate on the LCO smoothly without causing excessive pressure rise rate. On the other hand, the result shows that the heat release rate is significantly affected by the fuel injection quantity and timing. It implies that injection parameters for pilot fuel should be optimized in order to realize the reliable operation. Considering that the ignition behavior of pilot fuel heavily depends on fuel ignitability, it is expected that optimal injection parameters depend on the fuel ignitability. Therefore, in order to utilize pilot injection under several conditions for fuel ignitability, it is important to obtain deeper understanding on the roles of how pilot injection influences the improvement of the ignition behavior. This study investigates the effects of pilot injection on the ignition behavior using a rapid compression machine. Pilot injection parameters were varied systematically under different two ambient gas temperatures with two types of fuel. Shadowgraph was applied in order to obtain detail information on ignition process promoted by pilot injection. Visualization results provided detailed explanation on the roles of pilot injection in ignition event of main fuel.