The slope of velocity-time graph for motion with uniform velocity is equal to :

We should be aware that the motion of objects traveling in a straight line is described by the velocity-time graph. The graph may be used to calculate displacement and display acceleration. 

The slope of the velocity-time graph determines acceleration. A body's acceleration will be zero when it moves at a constant velocity. Therefore, the velocity-time graph will have a zero slope. For uniform velocity, the velocity will remain constant with respect to time. Hence the line representing the motion on the velocity-time graph will be horizontal. Therefore, the slope of the graph will be zero.