Abstract: Towards a list of rigid balls in inclined V-trough,experiments indicate that the motion shows some instability, namely, thesecond ball will be extruded from the V-trough when the initial number ofballs is odd, while the third will be extruded when the initial number iseven. Considering the column friction between contacts, numerical simulationswere made for this interesting phenomenon. The results areconsistent with experiments very well, thus leading toa few rules in rotation of balls. In the end,the relation between parameters and the degree at which a ball issubjected to extrusion are summarized according to experiments andnumerical simulations.