The strengh of two needle lockstitch seams was studied using the theory of the minimum loop strength of thread. In general, the strength of two needle seams F2 is expressed as follows: where θ is a variable which lays between 0 and 1 and is considered to be influenced by sewing factors such as thread combination, thread properties and consumption. Both F1 and F1′ are the theoretical strength, and F1 is the strength of a stitched line broken faster than another one in testing. In case of θ=1, two stitched lines in the seam will be broken at the same time and the seam will show the maximum strength in its combination of threads. F1 (and F1′) could be given by the following formula: where N is the number of loops in a stitched line, μ and σ, mean value and standard deviation of loop strength of thread respectively, and k, the correction term which mainly reflects the influence of loop angle at the interlacing part of the stitch. E(Rm) is the expected value of Rm which is defined as follows: where xm is the minimum loop strength. E(Rm) is calculated from the following equation: where Φ is the standard normal distribution function. In this study, k was assumed to be constant under our experimental condition and was given by the average value of k calculated from a case of single needle seams. θ was equated as the linear form using Δl which means the difference of the thread lengths per stitch between two stitched lines at the both breaking points. It was shown that the predicted strength of two needle seams agreed well with the experimental values.