Abstract [Co(phen) 3] 3+/multi-walled carbon nanotubes modified graphite electrode ([Co(phen) 3] 3+/MWNT/C) has been developed to monitor quantitatively 6-mercaptopurine (6-MP) concentration and to study the effect of rabbit blood as disturbance agents on the determination. Multi-walled carbon nanotubes (MWNT) are used for efficient immobilization of [Co(phen) 3] 3+. The prepared [Co(phen) 3] 3+/MWNT/C electrode exhibits an excellent redox activity. 6-MP is detected by monitoring the reductive current values of free [Co(phen) 3] 3+ or the [Co(phen) 3] 3+–6-MP complex generated by the interaction of [Co(phen) 3] 3+ on the MWNT modified graphite electrode with 6-MP. These electrochemical signals are found to be sensitive enough to monitor 6-MP concentration changes, and the [Co(phen) 3] 3+/MWNT/C electrode has also been applied to the evaluation of 6-MP in the presence of rabbit blood.