Compared with direct steam generation (DSG), direct vapor generation (DVG) system based on organic working fluid has better application potential in low and medium temperature distributed system. However, there is a lack of understanding of the two-phase flow instability that commonly occurs in DVG systems and can cause fatal damage to the system. in this paper, Ledinegg instability is considered as a common flow instability, and its occurrence characteristics and avoidance strategies are presented. First, a theoretical model was established to study Ledinegg instability of organic working fluid. Then, the effects of heat flux q, inlet subcooling T-c length-to-diameter ratio L/D and fluid properties are analyzed. Particularly, a characteristic parameter R-L representing the possibility of Ledinegg instability is proposed for the first time in performance evaluation of Ledinegg instability, The results show that as L/D decreases from 200 to 100, R-L reduces from 0.88 to 0.25. As q increases from 10 kW/m(2) to 30 kW/m(2), R-L increases from 0.54 to 1.03. When T-c is less than 3 degrees C, R-L approaches 0 and Ledinegg instability disappears. With the increase of surface tension a and the latent heat of vaporization r, the decreases of vapor-liquid density ratio rho(sv)/rho(sl) and the decreases of vapor-liquid viscosity ratio mu(sv)/mu(sl), R-L increases. The physical equation describing R-L of Ledinegg instability is helpful to guide DVG system to reduce or avoid Ledinegg instability in design and operation.