We have developed a conductive ink applicable to ink-jet printing to fabricate conductive lines on flexible substrates. Nano-sized silver particles having ∼ 20nm diameter was used for the direct metal printing. Silver conductive ink was printed on polymer substrates such as polyimide (PI), polyethylene naphthalate (PEN), and polyethylene terephthalate (PET) for the application to flexible electronics. The printing conditions of pulse frequency, pulse amplitude, xy-stage moving velocity and substrate temperature were optimized to achieve smooth conductive track with high resolution. After heat-treatment at temperature of about 100 - 300°C for 30 min, the printed silver patterns exhibit metal-like appearance and conductivity. The influence of the printing conditions on the microstructure and conductivity of the conductive track was investigated.