An ultrafast atomic motion of a photo-induced coherent phonon of bismuth at low temperatures was directly observed with time-resolved x-ray diffraction. A cryostat with a window that is transparent to both optical lasers and x rays enabled versatile diffraction measurements in a wide temperature range including below 10 K. It is found that an atomic displacement in a fully symmetric A_1g phonon mode is suppressed at low temperatures. This result indicates that the displacive excitation process is suppressed in the phonon generation with the decrease in temperature.