Long extendible spacecraft boom subjected to solar radiant heating is modeled as a thin-walled long cylinder of split non-overlapping section. Tip mass is attached to one end which is free to warp and other is fixed. The boom is heated by the unidirectional solar radiation assumed to be normal to its longitudinal axis. The equation of thermallyinduced torsional vibration of this system is formulated by considering the coupling effect of mechanical and thermal phenomena as one-degreeof-freedom system. The characteristic equation is evaluated using the Routh-Hurwitz stability criterion and it is found that the stability characteristics are dependent upon, along with three system parameters, the direction of radiant heating. The boundary curves, which divide the parameter plane into regions of stability and instability according to a direction of radiation, and some typical responses based on the closed-form solution are also included.