Directional solidification experiments were carried out at 0.5-150 $mu extrm{m}$/s in the Co-base superalloy ECY 768. As increasing solidification rate, the dendrite length increased and it reached the maximum at 150 $mu extrm{m}$/s, where the tip temperature is close to the liquidus. The liquidus and eutectic temperatures could be estimated by comparing the dendrite lengths and the temperature gradients at the solid/liquid interface and those were estimated as $1424.6^{circ}C$ and $1343^{circ}C$ respectively. Between the dendrites just below final freezing temperature, MC carbide and $M_{23}$$C_{6}$ carbide were found. It was confirmed that the script or blocky shape was Ta or W-rich MC carbide, and the lamellar shape was Cr-rich eutectic carbide. The solid/liquid interface morphology clearly showed that the Cr-rich eutectic carbide formed just after the script type MC carbide.