The high tenacity nylon 66 cord T4(A) was used instead of 2,100 dtex/2 nylon 6 cord in the carcass ply of 12.00-20 bias ply tire, and a comparative test was made.The heat resistance, dimension stability, and tenacity retention of nylon 66 cord T4(A) were better than those of 2,100 dtex/2 nylon 6 cord.The endurance, speed performance, strength, and tread life of bias ply truck tire were improved significantly by using nylon 66 cord T4(A), and the premature burst of tire bead was eliminated, thus the better economic benefit was obtained.