CHCl3 solution tend have a long stable jet, hence if you need a visible instability increase distance and/or voltage or decrease viscosity. Also option: add or use solvents with higher dielectric constant or conductivity.

I used combination of Chloroform:DMF (80:20), but it didnot worked. What do you suggest by raising the DMF ratio to almost 50 %? Will it work then?

DMF has a high B.P. Considering only PCL, I have experienced a stable electrospinning experience using only CHCl3 or CHCl3/DMF at a ratio of 75:25. Try to increase no more than 60/40. Nevertheless, maybe the problem is not being the solvent system but the blend concentration in solution and/or electrospinning parameters. Would you mind to share with us?

Voltage is 3 kV. Distance 16 cm. while blend concentration is PCL:PEO (80:20). flow rate, 1 ml/h ... Can you suggest some change?

Try a first experiment using: 15 kV, 10 cm as needle-collector distance and 0.5 mL h-1 as solution flow rate.! 

Ok. I will. thank you.

Do you think, blend ratio is fine?

It seems okay! Both polymers are usually easily electrospun. Which average molar mass? Which concentration (m/v) have you been using?

I increased the DMF ratio to 40, i.e. chloroform:DMF in 60:40 ratio. but now, particle formation took place.

If you change solution parameters you might need to change also some process parameter etc.

Please see the chart below as a first guidance to solve such challenges. 

http://electrospintech.com/espinflowchart.html#.WA8fS8lmfZ4