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The particle physics came out and developed with the exploration of the composition of the nature world.The Standard Model which was developed in early 1970s provides us a comprehensive and systematic theory to understand the particle physics.Many of the prophecies from the Standard Model have been proved by the experiments.Although the Standard Model has made such great successes, there are still many question need to be solved.The LHC, which is the largest high energy collider up to now, is constructed to provide the physics which can be used to test the Standard Model, and find the Higgs particle.It has 6 detectors, which are ATLAS, CMS, ALICE, LHCb, LHCf and TOTEM.Accurate study on the final reconstructed object performance is done to support all physics study.This thesis firstly presents my work on the electron identification efficiency study at ATLAS, which is used to improve the agreement of the electron performance between real data and simulated Monte Carlo. The LHC is a real top quark factory, as about 8 million top quark pairs will be produced per year.The top quark physics is an ideal physics channel to test the Standard model and find new physics, as the large mass and long life of the top quark.This thesis presents the measurements of the helicity fractions of the W boson and ttbar spin correlation in top quark pair semi-lepton decay, using the 4.7fb-1 data collected by the ATLAS experiment at LHC with proton-proton collision at the center of mass equal to 7TeV.In those analyses, I focus on the tt semi-leptonic channel (one W from top quark decays into leptons and another W from the second top quark decays into two light quarks).After the event selection, to further suppress W+jets and multi-jet backgrounds, I further require two jets to be tagged as bottom quarks.A complete reconstruction algorithm of the top quark pair system is described in the thesis.The physics variables are then calculated using the fully reconstructed tt system.The distributions are obviously biased by the hadronization/showering and detector effects, which are modelled by the matrices named the response matrices.The response matrices are calculated from Monte Carlo simulations.The unfolded angular distributions of final particles are then fitted by theory function to extract the Whelicity fractions and spin correlation.By using an unfolding method based on the migration probability from truth bin to reconstructed bin, the distributions of the angle are unfolded back to their parton level.After fitting the distributions with theory function, the W helicity fractions are measured:t(t) spin correlation differential result is also measured by using the unfolding method.Both of these two measurement results have good agreements with the Standard Model within the statistical and systematic errors.