More than 60% of the energy produced in the world is lost as heat, and about 90% of this heat is at low temperatures (below 200 °C). Given this waste of energy, any activity in the field of exploiting, reusing and converting it to other forms of energy is beneficial and valuable. Shape Memory Alloy heat engines convert low-temperature thermal energy into mechanical one. The operation of these engines depends on differences between the recoverable force generated at a high temperature (austenite phase) and the force as a result of deformation at a low temperature (martensite phase). Loading and unloading rates in heat engines are higher than quasi-static conditions. During the operation of these engines, some of the material properties also change as the sample’s temperature changes; therefore, the rate of loading and unloading directly affects the transformation kinetic of shape memory alloys. These conditions necessitate fully thermomechanical coupled models to study the thermodynamic behavior of these engines. To date, no comprehensive effort has been conducted on the theoretical study of the thermomechanical behavior and the efficiency of these engines. For this purpose, in this study, a fully-coupled thermomechanical model for analyzing shape memory alloy heat engines has been presented for the wire-shaped samples under tensile loads, in which the effect of temperature, stress and solid-state transformation have been considered. The model obtained in this study is able to analyze the behavior of these engines and to investigate various output parameters, including changes in stress, strain, temperature, output work, torque, power and efficiency under different conditions corresponding to three specific charactristics (i.e. transformation temperatures, Young's modulus of solid phases and critical transformation stresses), the surrounding environmental conditions (e.g. the type and temperature of hot and cold sources) and the structure of the heat engine (e.g. engine dimensions and number of the crancks). The properties of the Nickel Titanium have been used to numerically implement the present model in MATLAB software. By evaluating the influence of three aforementioned categories of arameters, a general model is presented to determine optimum operation conditions in order to extract the highest efficiency and output power. Keywords: Heat engine, Shape Memory Alloy, thermomechanics, Nickel Titanium, efficiency, output power