In this study, the K6509 cobalt-base superalloy were heat treated at different temperatures and the microstructure was detected by scanning electrical microscopy and electrical probe micro-analyzer. In as-cast K6509, the MC carbides are present as a discrete, blocky dispersion with a well-distributed Chinese script morphology and the eutectics (γ+M23C6) and (γ+M7C3) can be seen. With the temperature increasing, the MC arise degeneration. Below 1150oC, the script-like MC is dominant; reaching 1200oC, the particle and block of MC are dominant; reaching 1250oC, few script-like MC is present. In as-cast alloy, the M23C6 and M7C3 carbides are lamellar eutectic with matrix. When temperature is up to 1150oC, the eutectic (γ+M23C6) disappears; however, much eutectic (γ+M7C3) can be found. When the temperature reaches 1200oC, a large amount of eutectic M7C3 is dissolved, and some pores form in the matrix, and some zones rich in Cr form around the pores. When the temperature is above 1250oC, many pores form in the matrix, the cause of pores is need to be studied further.