From durability test, if we use conventional gasoline specified valve and valve seat for LPG liquid fuel injection system, we found that durability in unachievable. To improve this problem, it is more efficient to use conventional self-lubrication and high-hardness gaseous engine technology for exhaust-side. But we still found that we cannot solve this problem for intake side with the conventional technology. Because when injected LPG liquid fuel, it evaporates around the intake-valve and decreasing the temperature around it, and cannot built up oxidization layer, and consequence of adhesive wear. This problem can be solved surface treatment by soft nitride for the intake valve.