On September 1 st 2011, the Euro 5 + emission legislation will apply to passenger cars in Europe. It is characterized by the introduction of a new regulation concerning the number of soot particle emissions, whereas Euro5 legislation only applies to soot mass. This new regulation makes it necessary to investigate the impact of filter design on filtration efficiency as it pertains to particle number and to the Diesel Particulate Filter (DPF) operating window. This paper describes the investigation performed on a light duty test bench equipped with a Differential Mobility Spectrometer (DMS) which measures soot concentration number and soot particle size distribution in the exhaust gas effluent. Specific protocols were developed to be able to evaluate filtration efficiency in particle number when the DPF is empty of soot and also when the filter is soot loaded. Several DPF parameters such as filter length, filter diameter, cell geometry, and microstructure were studied. Moreover, in order to define the DPF operating window, severe active regenerations were also performed to determine the filtration efficiency in particle number as a function of DPF damage. Thanks to this ambitious test program, it was observed that the DPF operating window is impacted by filter design. The DPF thermal behaviour depends on filter size and microstructure. However, the DPF operating window is not significantly impacted when the filtration efficiency of particle number is considered in addition to filtration efficiency in mass.