1. A method for generating over-drive values applied to a 3D display device used with shutter glasses, the 3D display device comprising a first gray scale of a pixel for displaying a first-eye image and a second gray scale of a corresponding pixel for displaying a second-eye image, the method comprising the steps of: measuring a plurality of brightness values shown by using a plurality of first-eye gray scales in every n scales and a plurality of second-eye gray scales in every n scales at a first-eye channel of the shutter glasses; linearly interpolating the brightness values into N×N brightness values which consist of N first-eye gray scales respectively corresponding to N second-eye gray scales, wherein n and N are positive integers and N>n; calculating N×N crosstalk values of the N first-eye gray scales respectively switching to the N second-eye gray scales according to the N×N brightness values, wherein the N×N crosstalk values comprise a plurality of rising crosstalk values and falling crosstalk values; and determining a first over-drive gray scale and a second over-drive gray scale according to the N×N crosstalk values, such that a brightness value of a pixel being in the second over-drive gray scale and a brightness value of a corresponding pixel being in the first over-drive gray scale is equal to a brightness value of both the pixel and a corresponding pixel being in the first gray scale and such that a brightness value of the corresponding pixel being in the first over-drive gray scale and a brightness value of a corresponding pixel being in the second over-drive gray scale is equal to a brightness value of both the pixel and a corresponding pixel being in the second gray scale; wherein the step of determining comprises: selecting a first group of multiple falling crosstalk values less than a first numeric value among the falling crosstalk values, and then selecting a second group of multiple rising crosstalk values less than a second numeric value among the rising crosstalk values; computing sums of the rising crosstalk values to a power of p and the falling crosstalk values to a power of q for every pair of the first gray scale and the second gray scale from a plurality of pairs of the first gray scales and the second gray scales corresponding to the first group and the second group, wherein p and q are positive integers and p is larger than q; and selecting the first gray scale and the second gray scale corresponding to a minimum sum among the sums from the plurality of pairs of the first gray scales and the second gray scales to be utilized as the first over-drive gray scale and the second over-drive gray scale.