Degradation in wavelength accuracy and single-mode stability, incurred by random fluctuation of grating period, has been studied by an effective-index transfer matrix method in quarter wavelength shifted distributed-feedback lasers. The laser facets are assumed to be perfectly antireflection coated, and the period fluctuation is modeled as a Gaussian random variable. Laser wavelength variation originates from period-fluctuation induced wavelength change and threshold gain variation. Threshold gain difference decreases with increasing period fluctuation irrespective of grating coupling coefficient, while spatial-hole-burning effect is exacerbated or alleviated in low- and high-coupling coefficient devices, respectively.