Phase evolution and magnetic properties of melt-spun Nd8Fe86B6−xCx (x=0, 2, 4, 5, 6) alloy ribbons have been investigated. Increasing the C content was found to decrease the glass-forming tendency of as-spun ribbons. For samples with low C content (x⩽4), the best magnetic properties were achieved by directly quenching at an optimum roll speed, by which a uniform exchange coupled nanocomposite Nd2Fe14(BC)/α-Fe structure was developed. Moreover, within this composition range, C substitution did not significantly affect the microstructure of the optimally quenched ribbons. However, for the samples with higher C content (x>4), the composite 2:14:1/α-Fe structure can only be obtained by a phase transformation from the mixture of 2:17:Cx+2:14:1+α-Fe phases during annealing treatment. Moreover, the resultant 2:14:1/α-Fe composite structure was irregular, with a coarse grain size, which strongly degraded the exchange interaction between hard and soft magnetic phases. For optimally processed samples, replacement of up to 4 at. % of B by C significantly increased the coercivity with only nominal reduction in remanence, in contrast to the alloys with x>4, for which drastic deterioration of both coercivity and remanence occurred.