This paper presents experimental data on the critical heat flux (CHF) in twisted-tape-inserted tubes over a wide quality range of-0.25 to 0.8. The influences of quality, twist ratio, mass velocity, and clearance between the twisted tape and tube inner wall on CHF were investigated. In the subcooled region, it was observed, using an infrared thermoviewer, that CHF was initiated locally at the wall near the twisted tape. Consequently, twisted tape insertion with small tape-wall clearance decreased CHF to below the value in the empty tubes at a low flow rate. This decrease was found to be avoidable by adjusting the clearance. In the net quality region, CHF in the twisted-tape-inserted tubes increased with increasing flow rate contrary to the case of the empty tubes. However, CHF in the net quality region was also decreased by insertion of twisted tapes with high twist ratio (loosely twisted tapes) at a very low flow rate.