In this study, a numerical analysis was performed to investigate how the aspect ratio of flat tubes in a double-tube latent thermal energy storage system affects the melting behavior and thermal transfer properties of N-eicosane. To simulate the melting characteristics of the systems, the enthalpy–porosity method was employed. The aspect ratios of flat tubes in the inner and outer configurations significantly affected the melting characteristics. A smaller aspect ratio of the inner flat tube (AR i ) increased natural convection, resulting in a decreased total melting time. Conversely, a larger aspect ratio of the outer flat tube (AR o ) decreased the amount of solid PCM remaining, which also contributed to a shorter total melting time. For instance, in case 13, the total melting time was 1223.5 s, which represents a 59.3 % reduction compared with case 46, i.e., the base case with circular double tubes. The mean power in case 13 was 261.2 W, which corresponded to an increase of 146.08 % compared with the base case. Furthermore, at the optimal performance of AR o = 1.8, correlation equations for total melting time and mean power were derived to assess the influence of AR i regarding the melting process of systems.