The composite fin is presented based on the advantage of longitudinal vortex generator and slit fin, respectively. The performance of air-side heat transfer and fluid flow is investigated by numerical simulation for Reynolds number ranging from Re = 304 to 2130. Stepwise approximation method is applied on the mesh generation for the irregular domains of delta winglets and slit fins. The mechanism for augmenting heat transfer is also analyzed based on the local fluid field, field synergy principle and entransy dissipation principle. The computational results show that some eddies are developed behind the X-shaped slit and delta winglet, which produce some disruptions to fluid flow and enhance heat transfer; compared with plain fin and slit fin, it shows the composite fin has better heat transfer performance. By applying on the field synergy principle and entransy dissipation principle to analyze the composite fin, the computational results show that composite fin can improve the synergy of temperature gradient and velocity fields, and its equivalent thermal resistance is smaller and its irreversibility of heat transfer is lower. (C) 2014 Elsevier Ltd. All rights reserved.