Hi Sureshbabu,
The duration change could be the result of a number of factors.
Firstly, during your update, if you do not update the remaining days then the activity will slip by the factor in which you are updating.
Secondly, your duration change may be the result of adding resource, changing units over time or updating units for the resource. Check which duration type you are using and refer to the P6 manual for an explanation on the different scheduling options.
Your activity should not change due to the calendar you are using. The duration should remain constant from the time you assigned the calendar. Rescheduling will not change the duration. However, if you update the calendar with say public holidays, or revised shift patterns, this could affect the activity duration.
Hope this helps.
Simon