Residence time distribution is among the important parameters of the Buss Ko-Kneader which allows one to better understand the flow conditions inside the system and to get information on mixing efficiency. However it has always been a difficult task to predict the RTD of complex geometries such as Buss elements. The aim of the present work was to estimate the residence time distribution in EZ and KE element of the Buss Ko-Kneader by utilizing an elemental tracing method performed on the flow fields in the same elements which were analysized by CFD method. The effects of rotating and reciprocating motions of the screw have been taken into account in three dimensional flow analysis. It was demonstrated that these results can provide valuable information on mixing performance of these two elements.