Abstract: Abstract: A flat ring die mill is frequently used for producing pellets. In the densification process, a great amount of energy is required to avoid spring back and to overcome the friction between the material and the channel surface of the die. However, extra energy is unnecessarily consumed because of friction between densified material and the pressure between the roller and the die, where there are no opening channels. The aim of this work was to attempt to eliminate a portion of the frictional and compaction energy based on an improving method of densification using a flat die. A new flat die pellet mill with plunger was developed. The self-developed flat die pellet mill mainly consisted of three parts: a flat die, plungers, and a swash plate. When the swash plate rotated, it drove the rolling bearing which was set up on one side of the plungers 16 mm in diameter to reciprocate along with the sliding bearing. The moment the plunger reached to the end position without the urge of swash plate, the return spring began to push the plunger back to the other end position. With the reciprocating motion of the plungers, the raw material under the plungers was compacted precisely only on the spot of the opening channel of the flat die to avoid compaction and friction that exists in the conventional flat die. Then the pellets were squeezed out through open end pelleting cylinders connected to the flat die. The pelleting cylinder was 16 mm in diameter and 80 mm in length, so the ratio of the length to the diameter of the pelleting cylinder was 5. The pelleting cylinders can be changed, then they can suit for different raw material. Moreover, each pelleting cylinder was wrapped by a heating tape with a thermocouple to preheat the cylinder die to a certain temperature when necessary controlled by a temperature controller and monitor. A series of experiments on its feasibility were carried out at ambient temperature. Corn straw and acacia were used as the experimental material. Corn straw was milled into fibrous pieces at three levels of particle size (0-2, >2-4, and >4-6 mm) with moisture content of 15%, while acacia was milled into fibrous pieces at three levels of particle size (0-3, >3-5, and >5-7 mm) with moisture content of 15%. The results demonstrated the pellet mill exhibited a good feasibility and capacity, which met preliminary requirements of pelletization. When working at the speed of 50 rpm, the molding rate in all the trials were above 98%. The productivity of using acacia was greater than that of using corn straw because of its higher bulk density. Furthermore, the density values of all the pellets were over 1.1 g/cm3, and the mechanical durability values of pellets were above 97%. After 4 weeks' trials working, there were no obvious signs of wear on the plungers and the cylinder dies. The results above revealed the feasibility of the improving densification method, and the flat die pellet mill had the ability to produce the qualified pellets and to avoid the unnecessary wear and deformation on the flat die, which would be a new way for biomass densification technology.