A method for starting an internal combustion thermal engine of a hybrid vehicle having: the thermal engine provided with a drive shaft; at least one pair of driving wheels; a transmission, which transmits the motion from the thermal engine to the driving wheels and has a clutch; and an electric machine, which is mechanically connected to the clutch downstream of the clutch, so that the clutch is interposed between the electric machine and the thermal engine; the starting method includes the steps of: opening the clutch to separate the electric machine from the thermal engine; separating the electric machine from the driving wheels; making the electric machine rotate by operating the electric machine as a motor; and closing the clutch when the electric machine reaches a launch rotation speed.