The likelihood function is an appropriate target function for re®nement of molecular structures using ®ber diffraction data. However, its practical application to ®ber diffraction faces two signi®cant obstacles: (i) the intensities of layer lines in a ®ber diffraction pattern usually arise from the superposition of several terms, each equivalent to a crystallographic structure factor, thereby making the calculation signi®cantly more complex than for the crystallographic case; (ii) to describe a molecular structure at the atomic level based on ®ber diffraction data, the radial and phase parts of the atomic coordinates must be treated separately owing to the uniaxial symmetry of the structure. These issues are addressed here in order to derive equations of likelihood functions for ®ber diffraction. The special case of a single term on a layer line is treated ®rst followed by extension of the method to the multiterm case. A practical dif®culty in implementation of likelihood for the multiterm case is that each term has a different variance. An analytical technique is described that allows the conversion of the unequal-variance case to an equal-variance case. This makes it possible to express the likelihood by an explicit formula, allowing a direct implementation of the likelihood calculation. A cylindrically symmetric model is proposed for error distribution of the atomic coordinates in a helical structure. Variances and offset coef®cients of the contributing terms in the likelihood functions are expressed in terms of the variance of the atomic coordinates in the cylindrical reference system.