2001 kia optima ecu 15 amp fuse blows when key is on.unplug the fuel pump relay and fuse don't blow.

Do you hear the fuel pump run, when the ignition switch is turned to ON?

Check all under hood and under dash fuses with a test light or multimeter. Verify the wiring to the fuel pump, fuel pump relay, and ECM. Look for any shorts. You can try replacing the fuel pump relay and cleaning the main ECM ground, but my gut is that there may be a short in the ECM. ECMs can be damaged in many ways. You may need to look for a remanufactured ECM--shop for price and warranty, if necessary.

Did you every use jumper cables to start the engine?

Read this post and check for these wires

Kia blows ecu fuse

Definitely try RickMN's suggestion, which would be a simple, inexpensive solution.