In this study, the core concentration effect on diameter of core/shell electrospun fibers was investigated. The core/shell fibers were made by 10,12 and 14% (w/v) polycaprolactone (PCL) in chloroform and DMF (9:1 in volume) as core solution and 10% (w/v) polyvinylalcohol (PVA) in water as shell solution. These samples were examined under two conditions (1 and 2.5 flow rate ratio). The structure and morphology of fibers were surveyed by a field emission scanning electron microscopy (FESEM) and the diameter of fibers were measured. Three concentrations of core solution were evaluated whereas the shell concentration was kept constant. The FESEM micrographs showed that the overall diameter of electrospun fibers increased with increasing the concentration. Also the core concentration effect on conductivity and shear viscosity of core solution are reported. As a result, by increseaing the concentration, the shear viscosity enhanced and the conductivity changed negligible.