This is related to How to root the Xperia Mini ST15i.
I followed the instructions from the answer, but I'm stuck.
Now open the flashtool again and flash the stock kernel of ST15i and now enjoy the rooted phone. For check if you have rooted your phone successful there is an app installed superuser on your phone or you can download the Go backup/Titanium Backup to check out its rooted or not.
After flashing the stock kernel, the phone is stuck with the Sony logo, and keeps on restarting. The phone can't detected by the PC, the Sony PC companion, or the Sony Ericsson Update Service. Because it can't be detected, I can't flash it. It doesn't even charge when it's connected through the charger cable.