We investigated the dynamic viscoelasticity and elongational viscosity of polypropylene (PP) containing 0.5 wt% of 1,3:2,4-bis-O-(p-methylbenzylidene)-D-sorbitol (PDTS). The PP/PDTS system exhibited a sol-gel transition (T (gel)) at 193 degrees C. The critical exponent n was nearly equal to 2/3, in agreement with the value predicted by a percolation theory. This critical gel is due to a three-dimensional network structure of PDTS crystals. The elongational viscosity behavior of neat PP followed the linear viscosity growth function 3 eta(+) (t), where eta(+) (t) is the shear stress growth function in the linear viscoelastic region. The elongational viscosity of the PP/PDTS system also followed the 3 eta(+) (t) above T (gel) but did not follow the 3 eta(+) (t) and exhibited strong strain-softening behavior below T (gel). This strain softening can be attributed to breakage of the network structure of PDTS with a critical stress (sigma (c)) of about 104 Pa.