Darrieus turbine is a type of vertical axis wind turbines that unlike it's simple structure, behavior analysis is a hard computational task. Because of the complex flows around the machines, aerodynamic optimization problem that still remains an open question. In this paper, a numerical algorithm based on the Double Multiple Stream tube model is used to calculate the effect of the parameters that influence the efficiency of the Darrieus turbine. This method is a semi-empirical method using lift and drag coefficients obtained from experimental data. The comparison between the results of the present study with the experimental measurements shows that although the developed algorithm gives acceptable results, but, for higher rotational speeds gets than nominal rotational velocity, the model accuracy gets lower. The aim of this paper is to find optimal conditions, parametrically analyze the effect of blade thickness, solidity, Reynolds number, pitch angle and aspect ratio on turbine efficiency and start. The results show that increasing thickness, Reynolds number and solidity cause an increase in the turbine self-start capability. On the other hand, increasing the solidity of the turbine will reduce working range, and increasing the aspect ratio will increase efficiency especially at the nominal rotational velocity. The results also show that the designed turbine having variable solidity, can have the benefits of both low and high solidity turbines simultaneously. But manufacturing variable thickness blades doesn't have proper justification. Limited increase in pitch angle can also have positive effect on efficiency.