We studied the effect of thermal fluctuations on charge transport in two types of modified poly(dA)-poly(dT) DNA by calculating the transmission probability and I-V characteristics. The modification is carried out by replacing some of the AT base pairs with CG or GC base pairs placed randomly along the chain. The DNA model is placed in between two metallic electrodes. We used tight binding Hamiltonian to model the DNA molecule and electrodes. The thermal fluctuation is modeled by varying the twist angles of each base in the DNA. The transfer matrix and scattering matrix methods were employed in calculating the transmission probability. Transmission probability is used as an input in calculating I-V characteristics using Landauer-Büttiker formalism. The twist angle disorder is influenced by the temperature and frequency. In order to prevent spurious results, we take the average value transmission probability of 1000 realizations of twist angles. Our results indicate that the DNA modified by CG base pairs has its electrical properties impeded by increasing thermal fluctuation (increasing temperature and decreasing frequency). However, different behavior is observed on the DNA modified by GC base pairs, at low temperature its electrical properties are enhanced by decreasing frequency. Moreover, electrical properties of this DNA at high frequency are also enhanced as temperature increases.