Stabilization and control of the operating field mode in injection lasers are very important problems for wider application of the devices. This paper gives a theoretical analysis of the relation between transverse mode control and longitudinal mode behavior in the injection lasers which have undoped active regions. The guided modes are classified into three types, normal guided mode, active guided mode, and leaky mode, owing to guiding mechanism. Cutoff conditions for all types of modes are mentioned. Gain saturation was analyzed on the basis of the density matrix equation. The so-called weakly (partially) inhomogeneous property of the gain saturation was taken into account. The single longitudinal mode operation is expected when the transverse higher modes are in cutoff condition, because the suppression effect among the resonant modes is promoted. The normal guided mode is the most suitable for the single longitudinal mode operation, because the field profile and diffraction loss are invariable with increasing injection current. The transverse mode control is the most important characteristic in obtaining the single longitudinal mode operation in the injection lasers.