Method for operating a drive train of a motor vehicle, the drive train having an internal combustion engine and a double clutch transmission, the double clutch transmission having a first power transmission path with a first friction clutch and a first sub-transmission with odd gears stages and a second power transmission path with a second friction clutch and a second sub-transmission with even gear stages, and the internal combustion engine being coupled to an input member of the first friction clutch and to an input member of the second friction clutch, the drive train being a hybrid drive train which has an electric machine which is coupled to an output member of the second friction clutch, the method serving, starting from a state, in the case of which the internal combustion engine is switched off and therefore the rotational speed of the internal combustion engine is zero and, furthermore, the motor vehicle has a speed of greater than zero and in the case of which state the two friction clutches (20, 30) are open, to conduct drive power of the internal combustion engine via a lower gear stage in the second sub-transmission to an output, the rotational speed (n v) of the internal combustion engine being increased for this purpose before step a) in a passive way from zero to an output rotational speed (n 1) which is greater than zero by the friction clutch which is assigned to the first sub-transmission being closed with slip, the gear stage which is selected in the first sub-transmission being a gear stage here which is sufficient to spin up the internal combustion engine to its ignition rotational speed, the drive train being operated, furthermore, in such a way that, for the time period, within which the rotational speed (nv) of the internal combustion engine is increased to the output rotational speed (ni) in a passive way before step a), the electric machine transmits a drive torque via the second sub-transmission to the said output, and with the following steps: a) increasing of the rotational speed (n v) of the internal combustion engine, starting from the said output rotational speed, b) engaging of the lower gear stage in the second sub-transmission, and engaging of a high gear stage in the first sub-transmission, if it has not yet taken place, c) renewed closing with slip of the friction clutch which is assigned to the first sub-transmission, as soon as the rotational speed (n v) of the internal combustion engine has reached an intermediate rotational speed (n 2) which corresponds to the high gear stage in the first sub-transmission, in order to conduct drive power via the high gear stage to the output, and d) overlapping closing of the friction clutch which is assigned to the second sub-transmission and opening of the friction clutch which is assigned to the first sub-transmission, in order to conduct drive power via the lower gear stage to the output, as soon as the rotational speed (n v) of the internal combustion engine has reached a target rotational speed (n 3) which corresponds to the lower gear stage in the second sub-transmission. a) increasing of the rotational speed (n v) of the internal combustion engine, starting from the said output rotational speed, b) engaging of the lower gear stage in the second sub-transmission, and engaging of a high gear stage in the first sub-transmission, if it has not yet taken place, c) renewed closing with slip of the friction clutch which is assigned to the first sub-transmission, as soon as the rotational speed (n v) of the internal combustion engine has reached an intermediate rotational speed (n 2) which corresponds to the high gear stage in the first sub-transmission, in order to conduct drive power via the high gear stage to the output, and d) overlapping closing of the friction clutch which is assigned to the second sub-transmission and opening of the friction clutch which is assigned to the first sub-transmission, in order to conduct drive power via the lower gear stage to the output, as soon as the rotational speed (n v) of the internal combustion engine has reached a target rotational speed (n 3) which corresponds to the lower gear stage in the second sub-transmission.