The β-decay constant of ¹³⁸La has been geochronologically determined using Archean granites from the Yilgarn Block, Western Australia. Rb-Sr and Sm-Nd mineral isochron ages were obtained for the two granite samples, which are nearly identical with the ion microprobe U-Pb zircon ages. This agreement indicates that the closure date for these samples is the same, within analytical error, making them suitable for geochronological estimation of the β-decay constant for ¹³⁸La. Applying a new method of Ce isotope analysis, with analytical reproducibility of ±0.008%, La-Ce isotope data for the studied samples resulted in precise mineral isochrons. Based on these internal mineral isochrons and ages determined by other geological clocks, the β-decay constant for ¹³⁸La was estimated to be (2.33 ± 0.24)·10⁻¹²a⁻¹ (2σ).