Binary stars play an important role in the evolution of stellar populations . The intrinsic binary fraction (f (bin)) of O and B-type (OB) stars in LAMOST DR5 was investigated in this work. We employed a cross-correlation approach to estimate relative radial velocities for each of the stellar spectra. The algorithm described by Sana et al. (2013) was implemented and several simulations were made to assess the performance of the approach. The binary fraction of the OB stars is estimated through comparing the uni-distribution between observations and simulations with the Kolmogorov-Smirnov tests. Simulations show that it is reliable for stars most of whom have six, seven and eight repeated observations. The uncertainty of orbital parameters of binarity becomes larger when observational frequencies decrease. By adopting the fixed power exponents of pi = -0.45 and kappa = -1 for period and mass ratio distributions, respectively, we obtain that f(bin)=0.4(-0.06)(+0.05) for the samples with more than three observations. When we consider the full samples with at least two observations, the binary fraction turns out to be 0.37(-0.03)(+0.03). These two results are consistent with each other in 1 sigma.