The longitudinal magneto-transport in narrow-gap semiconductor InSb under a quantum magnetic field at 42 K is investigated using the balance equation theory. The nonparabolicity of the energy band and scatterings due to ionized impurities, acoustic and polar optical phonons are taken into account.All the subbands occupied by electrons are included in the calculation. The longitudinal magnetic field reduces the drift velocity and leads to a decrease of the electron temperature. The drift mobility decreases with increasing magnetic field. The drift velocity under the extreme-quantum-limit assumption is completely different from that taking into account all possible subbands occupied by electrons.Hot-electron transport in the presence of both an electric field and a magnetic field has been an interesting problem for a long time [l to 41. The balance equation theory [5] has been used to study the magneto-transport in narrow-gap semiconductors under the influence of a weak magnetic field [6, 71, where the Landau quantization of the energy bands is neglected. The balance equation theory predicted that the Hall factor in semiconductors is sensitive to the band structure and is equal to unity in parabolic bands and less than unity in nonparabolic bands, which is in better agreement with experiment [8] than the result of Monte Carlo simulation [9]. These predictions, however, are not valid when the magnetic field is strong enough.Under the influence of a large magnetic field, the conduction band breaks into Landau levels. The quantum effect becomes more prominent in narrow-gap semiconductors due to the small effective mass. In the last decades, considerable attention has been paid to the hot-electron transport in narrow-gap semiconductors in the extreme quantum limit [lo to 131, where only the lowest Landau subband is assumed to be occupied by electrons. However, this assumption is valid only at low temperature and under large magnetic field. For the case of medium magnetic field and higher temperature, this assumption is inappropriate.In this paper, by using the balance equation [5], we present the theoretical calculation of the longitudinal magneto-transport in the narrow-gap semiconductor InSb at 42 K when the magnetic field varies from medium to large values. The nonparabolicity of energy bands and the scatterings due to ionized impurities, acoustic, and polar optic phonons are taken into account in the calculation.