Abstract Coiled Tubing (CT) is sometimes used in applications where the pressure outside the CT may be higher than the pressure inside the CT. These applications include operations in high pressure wells1 and reverse circulation operations where a fluid is pumped down the annulus and returns are taken up through the CT. If the difference between the internal and external pressures becomes too large, the CT will collapse. CT collapse could lead to serious well control problems. During its life CT becomes oval due to bending around the reel and over the gooseneck. This ovality reduces the collapse pressure of the tubing. The pressure and tension limits for CT were analyzed in reference 2. The need for further work to understand the affect of ovality on the collapse pressure of CT was mentioned and is the purpose of this paper. First an analysis was made to develop an analytical method for calculating the collapse pressure of an oval tube. Then testing was performed to validate this method.