The probability of finding electrons along the DNA chain is described by localization length. We use tight-binding Hamiltonian approach, transfer matrix, and Gram-Schmidt orthogonalization method in studying the localization length of poly(dA)-poly(dT) DNA molecule. The molecule being studied consists of 102 base pairs of adenine (A) and thymine (T) as well as backbone which consists of phosphate and sugar. Electron hopping between backbone is allowed in this model. The effect of magnetic field on the localization length is studied by considering the magnetic field influence on hopping electron in DNA in the form of Peierls phase factor. The effect of temperature is studied by considering the twisting vibration of DNA molecule. The result shows localization length decreases when the temperature increase. On the other hand, when magnetic field increases localization length decreases in some energies.