High-pressure coiled tubing applications pose many technical challenges to coiled tubing string design: collapse, diametral growth, elongation and wall thinning, to name a few. This paper discusses an optimal string design methodology for coiled tubing in high pressure applications. The string design methodology uses a sophisticated collapse-prediction model for oval coiled tubing, taking into account the effect of axial load on collapse resistance. It also considers the effect of diametral growth, elongation and wall thinning on the performance of a high-pressure coiled tubing (HPCT) string. By integrating these design parameters into the framework of an optimal string design methodology, this paper presents an effective approach for high pressure coiled tubing string design. The paper also presents examples of HPCT string design. It is demonstrated that it is possible to design HPCT strings capable of withstanding more than 9000-psi collapse pressure while maintaining the field usability of the HPCT string.