Terahertz emission from BSCCO single-crystal mesa structures has recently attracted considerable attention. Since power fed into the mesa is of the order of 10⁶ W/cm³, a significant self-heating effect appears in I-V characteristics. We measured I-V characteristics of a BSCCO sub-THz oscillator using three cooling methods. When the mesa is cooled directly by liquid helium, the maximum applied voltage is increased up to three times compared with that cooled using a helium flow cryostat. Moreover, radiation characteristics of the BSCCO sub-THz oscillator in liquid helium were investigated using a BSCCO mesa detector. The radiation, identified as TM (2, 0) mode (i.e., the second harmonics of cavity resonance mode along width), is successfully observed.