Service tax on labour charges

Dear Sirs,

         We are a Fabrication unit doing fabrication with our own purchased raw material wherein we charge Excise Duty.

We also undertake fabrication jobs on Labour Basis, i. e. Our Client Supplies raw material under challan 4(5).

We would like to know wheather we are liable to charge Service Tax on

the Labour Bill so raised ?

Please Help me...

THANX In Advance...