To explore the influences of the angle of attack and the aspect ratio of a winglet, which is punched near the leading edge of the fin in a high performance finned oval tube (FOT), on the heat transfer enhancement (HTE) and Bow loss penalty (FLP), three-dimensional Bow and conjugate heat transfer in a FOT were calculated for a thermally and hydrodynamically developing laminar flow (Re = 300) by solving the Navier-Stokes and energy equations with a Finite-Volume Method in body-fitted grids. The conjugate heat transfer was realized by iterations of the energy equation in the Bow field and of the conduction equation in the fin. Three angles of attack (beta = 20 degrees, 30 degrees and 45 degrees) and two aspect ratios (Lambda = 1.5 and 2) were investigated. Velocity and temperature fields, vortex formation, local heat transfer distributions and global results are presented. The winglet with beta = 30 degrees and Lambda = 2 provides the best ratio of HTE to FLP with (j/j(0))/(f/f(0)) = 1.04.