Abstract: The process capability index (C_PK) is the value that measures the degree of process capability to meet the quality standard of products. In the process control, if the C_PK is bigger, the product quality is better. Considering the economy of production and processing, the C_PK is usually controlled at an acceptable level. In this paper, we firstly applied a process capability index from industry to the quality assessment of analysis in the geological laboratory. The comprehensive evaluation of the overall quality of the samples was obtained through calculating the process of 114 capability indexes of the standard sediments (GBW 07320, GBW 07325 and GBW 07328) with the geochemical samples selected from the two Tibet 1∶200000 Luolamu map. The results show that all the C_PK were more than the threshold levels of C_PK. In all of the testing items, 63 items′ C_PK such as Na₂O, MgO, Al₂O₃ and SiO₂ etc, were more than 1.33, 47 items′ C_PK such as Cu, Li, Be, W etc were more than 0.67. However, the C_PK of Th, Ag, Cr and Hg were less than 0.67. As for the items with poor process capability, the tests for process capability were taken, appropriate standard materials were selected, equipment calibrated, chemical procedures standardized and the measurement methods optimized in order to improve accuracy and stability of the analytical data. The application of process capability index provided a convenient and scientific method for quality evaluation of geochemical sample analysis, which provided the scientific basis to choose analysis plans for different testing items and reduce the cost.