Abstract: The discovery of X-ray, as that of its precursor the cathode ray, is the most influential, and serendipitous, event in history of science. In order to extract cathode ray from a Crookes tube, Röntgen applied a relatively higher voltage and carried out the experiment in dark, thus unintentionally found a novel unknown ray, which he named the X-ray. Röntgen was awarded the first Nobel prize in physics for his discovery of X-ray, which in turn has brought with it a lot of new findings and benefits to the human society.