The effect of a ceramic diesel particle trap on the size and number of particles emitted from an IDI diesel engine was investigated. Transient effects on the distribution were revealed by utilizing an Electrical Low Pressure Impactor for real time measurement of the particle distribution. A conventional and a low sulfur - low aromatics fuel were used with and without Cerium based additive. Both parameters considerably affect the number of emitted particles, while the median size, expressed as the aerodynamic diameter equivalent, is found in the range of 60 to 110nm. Furthermore, a major influence of the trap load on the total particle number was detected during the accelerations of an urban driving cycle.