Abstract： Observation of Higgs boson production in association with a top quark pair (ttH) is presented, based on the analysis of up to 79.8 fb 1 of proton-proton collision data produced at 13 TeV center-of-mass energy by the Large Hadron Collider and recorded with the ATLAS detector. The results for Higgs boson decay channels of bb, diphoton, WW, ditau, and ZZ are combined. The observed significance is 5.8 standard deviations, compared to an expectation of 4.9 standard deviations. Combined with the ttH searches using a dataset corresponding to integrated luminosities of 4.5 and 20.3 fb 1 at 7 and 8 TeV, respectively, the observed (expected) significance is 6.3 (5.1) standard deviations. Assuming Standard Model branching fractions, the total ttH production cross section at 13 TeV is measured to be 670 ± 90 (stat.) +110/-100 (syst.) fb, while the Standard Model prediction is 507+35 50 fb. CERN issued a press release for both the ATLAS and CMS ttH results on June 4th 2018. Moreover, this talk will give detailed description of the new ttH (H→gamma gamma) analysis. Applying advanced machine learning package XGBoost BDT, exceptional performance is achieved by using object level kinematic variables from jets, leptons, and photons to separate the ttH signal from the background.