Refer Section 70 of Finance Act, 1994
(1)] Every person liable to pay the service tax shall himself assess the tax due on the services provided by him and shall furnish to the Superintendent of Central Excise, a return in such form and in such manner and at such frequency [and with such late fee not exceeding[twenty thousand rupees] rupees, for delayed furnishing of return,] as may be prescribed.
[(2) The person or class of persons notified under sub-section ( 2) of section 69, shall furnish to the Superintendent of the Central Excise, a return in such form and in such manner and at such frequency as may be prescribed.]
The dept. has explained this provision in detail in its Circular dated 23/08/2007 which may be referred to.
It may be noted that in para 6.1
6.1 The service tax return is required to be filed under Section 70 of the Act read with rule 7 of the Rules, by 'any person liable to pay the service tax'. This return is required to be filed on a half yearly basis, in Form ST-3. For the periods from April to September and October to March, it must be filed by the 25th October and the 25th April respectively. Further, 'Input Service Distributor' is also required to file this return. Persons who are not liable to pay service tax (because of an exemption including turnover based exemption), are not required to file ST-3 return.
So it is very clear tht an assessee who has opted to avail of the threshold exemption and has not crossed the maximum limit of Rs. 10 lacs during the F.Y need not file a return.