In present work we study the unification of classical general relativity (GL) and quantum mechanics (QM) which results in a new limiting dimensional quantity named as “Planck Length”. The search of line of demarcation between these two theories gives the idea to define this new minimum possible length. Planck’s Length (LP) is defined in terms of universal gravitational constant G, Planck’s constant h (or ћ=h/2π) and velocity of light c. Physical significance of plank’s length is also discussed.