An uncertainty relation of space-time

We propose an uncertainty relation of space-time. This relation is characte rized by GhT less than or similar to delta V, where T and delta V denote a characteristic time scale and a spatial volume, respectively. Using this un certainty relation, we give qualitative estimations for the entropies of a black hole and our universe, We obtain qualitative agreements with the know n results. The holographic principle of 't Hooft and Susskind is reproduced . We also discuss cosmology and give a relation to the cosmic holographic p rinciple of Fischler and Susskind. However, as for the maximal entropy of a system with an energy E, we obtain the formula root EV/Gh(2), with V denot ing the volume of the system, which is distinct from the Bekenstein entropy formula ER/h with R denoting the length scale of the system.