Abstract: Being aimed at the method which taken "x±3σ" as the control limits of control charts in quality control,took the x-R control charts for an example,a new opinion of the optical limits was pointed out through the analysis of the variation between the control limits of control charts (Kσ) and the probabilities as well as the costs of the two types of the errors (α and β).We established a mathematical model for its costs,gave out the major parts of the researching process and the optical formula for the control limits of control charts (K),and made a further discussion and extension for the generalization as well as the advantages of its results by the comparison with the results with x±3σ method.

Key words: quality control charts, the first type of error (α), control limits, the second type of error (β)