A series of Co-doped BaFe2-xCoxAs2 (0 <= x <= 0.32) single crystals have been grown by self-flux method. Study of X-ray diffraction indicates that the lattice parameter c of the crystals decrease linearly with the increase in Co-content added to the initial mixtures. The segregation coefficient of Co related to Fe is determined to be similar to 0.78. The values of superconducting transition temperature T-c are found to increase to about 1-3 K by post growth annealing at high temperature, which should result from the improved crystallinity due to the release of residual strain in the samples. In addition, we demonstrate that the annealing atmosphere of Ar, air or vacuum has an identical effect on the enhancement of T-c. Due to inhomogeneous Co distribution, a broad transition temperature width Delta T-c is observed and discussed for the over-doped samples. (C) 2011 Elsevier B.V. All rights reserved.