The Fundamental postulates of kinetic theory of gases are:

i) All gases are composed of small individual particles called molecules. The molecules of a gas is same and the  molecules of different gases are different.

ii) The molecules are assumed to be perfectly elastic spheres and there is no mutual attraction or  repulsion among the molecules.

iii) The molecules are continually in motion and this motion is completely random. i.e., they are moving in all possible directions with all possible velocities ranging from zero to infinity.

iv) During random motion, the molecules of gas collide with each other and the walls of the container continuously. Between any two collitions, the molecules travel in straight line. The distance between any two consecutive collitions is called the free path and average value of the free paths are known as mean free path.

v) The time of collision to occur is negligible in comparison to the time required to traverse the free path.

vi) Since, the molecules are elastic sphere, the momentum and energy of the molecules before and after collisions remains same. That is, the motion of the molecules obeys the conservation of momentum and energy.