An online ultrasensitive method for detection of nickel ions by capillary electrophoresis-post chemiluminescence with field-amplified sample injection has been developed. Under the optimal conditions, the detection limit for Ni2+ was 1.0 x 10(-14) mol relative standard deviations of peak area and migration time using 1.0 x 10(-12) mol L-1 Ni2+ (n = 7) were 6.23% and 2.86%, respectively. This is a lower detection limit for Ni2+ than previously reported. The proposed method was successfully applied to the separation of Hg2+, Ni2+, and Cu2+ in a mixture.