An idealized model for a twisted yam structure assumes that each fiber or filament follows a helical path and the yam has a circular cross section in the undeformed state. An investigation of the behavior of a single helix can therefore be considered as a first step toward the mechanical analysis of yam deformation under simultaneous extension, compression, and bending, which is the kind of deformation that occurs in many textile processes and applications. This paper presents a theoretical model of the large-scale deformation of a single helix subjected to bending, compression, and extension. The work studies the effects of varying the ratio of the flexural rigidity to the torsional rigidity of the helix material and of varying the helix angle together with the variation of the deformation forces. The cross section of a deformed helix is discussed in relation to the position of the neutral axis and strain energy. Also, some experimental data are presented to show the relevance of the theoretical results to the experimental findings.