Principle of minimum potential energy (PMPE) as one of the fundamental concept in physics and engineering will be used in this work to explain the phenomenon of buoyancy, why object with lower mass density can float on fluid with higher mass density, while object with higher mass density can sink in fluid with lower mass density. Before that some examples in mecahnics, e.g. object with different vertical position, object on incline, object with spring moving in vertical direction, and final condition of seesaw-like system, will be discussed to show the consequence of PMPE. Then, in the system of solid object and fluid, including the surrounding fluid with the object and treat them as a whole object or system we can easily show that the floating or sinking phonomena are simply a state where system potential energy is miniumum (both object and fluid). For simplicity, the theoretical demonstration will only use the form of block, where the other form, e.g. cylinder, sphere, cone, irregular form, are the matter of geometry difficulty only. Students engagement to the topic has not been performed but the instrument is proposed for future work.