In this paper, an extended car-following model with consideration of the combination effect in heterogeneous urban traffic flow with slow and fast vehicles is proposed. The combination effect means the order of vehicles in a heterogeneous flow matters, since a fast vehicle would degenerate to a middle state between a fast vehicle and a slow vehicle when it follows a slow vehicle. Specifically, we consider three types of vehicle combinations in the urban traffic flow, namely fast vehicle following fast vehicle (FF), fast vehicle following slow vehicle (FS), and slow vehicle following fast/slow vehicle (SX). Linear stability analysis is conducted to obtain stability criterion, from which we can conclude that the higher penetration of the FS combination can increase the stability of traffic flow.