Without modifying the General Electric XRD-5 diffractometer accurate lattice parameters of cubic structures may be determined quite simply and precisely. The method is applicable to both solid and powder samples and gives a precision of at least ±0.00025 A. The most important step in the analysis is the alignment of the diffractometer. The specimen must be in the exact center for precise work. However, after the initial alignment, daily alignment is a routine matter. This technique for lattice-parameter determination will also be useful for determining thermal coefficients of expansion and residual-stress measurements.