The shape, size, and content of MC carbides in a unidirectional Ni-base superalloy have been studied at different solidification rates using material of normal commercial composition (5 ppm nitrogen) and two melts with additions to produce 22 and 30 ppm nitrogen. M(C,N) was found in nitrogen-doped specimens. The increased nitrogen content results in a change in carbide morphology from acicular or Chinese-script type to a blocky one. Carbide size increases from 6.85 to 7.25 µm at 2.5 µm/s and from 2.31 to 2.81 µm at 200 µm/s as the nitrogen content increases from 5 to 22 ppm; carbide size is reduced to 5.2 µm at 2.5 µm/s and to 2.1 µm at 200 µm/s when nitrogen content is further raised to 30 ppm. There is no evident change in carbide content (area percentage) with increase of nitrogen. Scanning electron microscopy (SEM) and electron probe microanalysis revealed characteristic centers in some of the blocky carbides of TiN, as nuclei of MC or M(C,N) formed in the melt. The influences of nitrogen on carbide at the low solidification rate are stronger than those at the higher rate. With the increase of solidification rate, the carbide turns from bar to Chinese-script type, except for the base alloy at 2.5 µm/s, and the content and size decrease.