Method - 2

Requirements:

• Charge your Xperia U to at least 50% so there is no power interruption during this process.
• Make sure it is running Official ICS 4.0.4 and not any other firmware such as Gingerbread 2.3.7
• Download Flashtool

Since rooting is not yet possible on the 6.1.1.B.1.89 kernel, users have to first flash the 6.1.1.B.10 kernel which can be found .here

This method only work with Xperia U ST25i with firmware 6.1.1.B.1.10

1. Switch USB-Debugging on:

    Setting >Developer Option >USB-Debugging

2. Install Flashtool on your pc

   Download Link - Flashtool

3. Instal Driver from following Directory

    C:\Flashtool\drivers\

   Choose Xperia U from mention list

                              

   

4. If Your ST25i already Bootloader unlocked then proceed .If not then see my next tutorial how to unlocked Bootloader .

4.1 Now open Flashtool

4.2 There u See Locked type Icon , Which indicate "Root Device"

My handset is alredy rooted so the locked icon look faded ..but if not then it seems like black icon

4.3 Now select Type of root .It's 

    -Super User

    -Super SU

which ever u like 

4.4 At this process your phone restart several time .

      You have to select "Restore"   When they asked in Process

It's Done your phone is rooted