Abstract: Two-roll cross rolling piercing mill is widely used in the hot rolling. Seamless tubes. In the past it is considered that the piercing process is "surface contact" between the, barrel shaped rolls and the piereing-rolled metal, However, this is not a scientific concept, The writer of this article has found that there is a disconnected contact curve on the contact surface"). Thus, the conjngate contact drive is formed through the contact curve. For putting forward the designing theory of compound curve roll the writer adopts the linear contact conception in the article, and the producing practices onφ76φ100 two-roll cross rolling piercing mills have proved that designing and computing are correct. The article recommends that the design of all Mannesmann-type piercing mills may adopt the compound curve roll, which provides a steady piercing and strengthening piercing conditions, thereby greatly increasing the productivity and improving the shell guality.