A univibrator circuit using a tunnel diode and a transmission line is analysed. A graphical method is presented to calculate the width and the magnitude of the univibrator pulse. Conditions for the univibrator action to take place are derived. The effects of variation of the tunnel diode bias and the characteristic impedance of the line on the pulse width are also studied. The modifications to be made in the graphical analysis for the case of a line with small losses are discussed in detail.