In the recent years diesel engine developers and manufacturers achieved a great progress in reducing the most important diesel engine pollutants, NOX and particulates. But nevertheless big efforts in diesel engine development are necessary to meet with the more stringent future emission regulations. To improve the knowledge about particle formation and emission an insight in the cylinder is necessary.By using the fast gas sampling technique samples from the cylinder were taken as a function of crank angle and analyzed regarding the soot particle size distribution and the particle mass. The particle size distribution was measured by a conventional SMPS. Under steady state conditions the influence of aromatic and oxygen content in the fuel on in-cylinder particle size distribution and particle mass inside a modern 4V-CR-DI-diesel-engine were determined.After injection and ignition, mainly small soot particles were formed which grow and in the later combustion phase coagulate. The soot is formed very rapidly in the first combustion phase. After that soot particle oxidation takes place. In the first oxidation phase mostly the larger particles were oxidized. In the later expansion phase all soot particles were oxidized at nearly equal rate with a tendency of higher oxidation quantity for larger particles. The particle size distribution at the end of the expansion phase is similar to the tailpipe particle size distribution. No influence of the fuel composition on the in-cylinder and tailpipe particle size distribution was found.