The space-time description in Physics was composed of 3D space and the 1D time which was independent of 3D space before the proposal of the 3D space-time. Therefore, space-time was regarded as 4-dimensional. With the metrical method, the author finds out that time and space can be quantified by the same quantity value which can be described by three numbers (coordinate values) if the metric for the space distance is selected as the time wave length. Hence, space-time can be regarded as not only the observable quantity but also 3 dimensional.
