We investigate the filament thinning behavior of the suspensions under extensional flow in the final stage of filament breakup process. Under the extensional flow, the particles dispersed in a Newtonian medium induce complex behaviors; in particular, the particles cause non-uniform filament thinning when the minimum neck diameter (W) of the filament becomes as thin as the length scale of the particle. We suspend 20 wt% of 10 mu m poly(methyl methacrylate) (PMMA) particles in a silicone oil, and compare the filament thinning of the suspension with that of silicone oil having the same viscosity and surface tension. In this way, the effect of the particles on the thinning filament can be confirmed while eliminating differences in the rheological properties of the two fluids. The particles begin to affect filament shape when the filament becomes thinner than W = 10 D (D = 10 mu m; particle diameter). The filament surface is rough and the shape of the filament is random for W < 2 D, which leads to a random pinch-off of the thinning filament, because the particles confined in the filament are distributed non-uniformly. The random distribution of the particles induces heterogeneity in the final stage of the filament thinning process, when the filament becomes as thin as the length scale of the particle.