Optimal precursor perturbations of El Niño in the Zebiak-Cane model were explored for three different cost functions. For the different characteristics of the eastern-Pacific (EP) El Niño and the central-Pacific (CP) El Niño, three cost functions were defined as the sea surface temperature anomaly (SSTA) evolutions at prediction time in the whole tropical Pacific, the Niño3 area, and the Niño4 area. For all three cost functions, there were two optimal precursors that developed into El Niño events, called Precursor I and Precursor II. For Precursor I, the SSTA component consisted of an east-west (positive-negative) dipole spanning the entire tropical Pacific basin and the thermocline depth anomaly pattern exhibited a ten-dency of deepening for the whole of the equatorial Pa-cific. Precursor I can develop into an EP-El Niño event, with the warmest SSTA occurring in the eastern tropical Pacific or into a mixed El Niño event that has features between EP-El Niño and CP-El Niño events. For Precur-sor II, the thermocline deepened anomalously in the east-ern equatorial Pacific and the amplitude of deepening was obviously larger than that of shoaling in the central and western equatorial Pacific. Precursor II developed into a mixed El Niñoevent. Both the thermocline depth and wind anomaly played important roles in the development of Precursor I and Precursor II.