We have developed a conductive ink applicable to ink-jet printing to fabricate conductive lines on flexible substrates. Nano-sized silver particles with similar to 21 nm diameter were used for the direct metal printing. Silver conductive ink was printed on polymer substrates for the application to flexible electronics. The printing conditions of pulse amplitude, head frequency, xy-stage moving velocity and substrate temperature were optimized to achieve smooth conductive track with high resolution. After heat-treatment at temperature of 100 similar to 300 degrees C for 30 min, the printed silver patterns exhibit metal-like appearance and high conductivity. The influence of the printing conditions on the microstructure and conductivity of the conductive track was investigated.