The quantitative analysis using laser induced breakdown spectroscopy (LIBS), lack of appropriate interior label element, is described and applied to trace element molybdenum (Mo) detection in complex metallic alloys. A Q-switched Nd∶YAG laser operating at 532 nm was utilized to generate plasma and the emission was recorded by a grating spectrometer equipped with CCD, boxcar and PMT. The three peak heights of Mo Ⅰ, 550.649, 553.305 and 557.045 nm, changing with Mo mass fraction in metallic alloys were measured to produce calibration curves respectively, and double blind method was used to analyse a test sample. Based on Mo Ⅰ 550.649 nm line, the Mo mass fraction in the test sample was determined to be 2.229% with relative error of 5.57% in comparison with the given value of 2.111%. On using Mo Ⅰ 557.045 nm line, the relative error was found to be doubled due to the overlap with Fe emission lines. While taking the total height of three atomic lines into account in analysis, the resulted error dropped to 7.58%, even better than the average of predicted concentrations based on the above three lines. The obtained results demonstrate that satisfactory precision could be obtained under a consistent experiment condition with the above scheme, even without appropriate interior label element. The methods of maintaining stable laser ablation efficiency on sample are also discussed.