When i drag and drop material on to my particle system. It is emitting cubes instead of sphere ?

That is because the default particle is of a circular gradient, which means the center has an opacity of 100%, but the edges have an opacity of 0. The easiest solution is to do this:

void Start () {
		ParticleSystem renderer = this.GetComponent<ParticleSystem> ();
		renderer.startColor = Color.blue;
	}

Or

1. Create new Material
2. Change the shader to ‘Particles / Alpha Blend’ then change the tint color to whatever you like

3. Then assign this material to the ‘Material’ component inside of the ‘Renderer’ tab.

Hope this helps!

TBART82