The experimental study was carried out to clarify the low-quality dryout phenomena in helical coiled tubes placed horizontally. The results obtained are as follows. (1) The low-quality dryout is caused by the separation of liquid and vapor phases and it disappears at the transition from the separated flow to the slug flow regime. (2) The low-quality dryout generally takes place in the lower portion of a non-vertically placed helical coiled tube. (3) A criterion for disappearance of the low-quality dryout has been proposed based on the experiment.