if i have multiples data flow tasts in a  ackage, and i want to make sure only i data flow is executed, how do i cntrol at job level.

Hi,

I could not completely understand your exact requirement ,

but if want to handle package execution in such a way that only if all DataFlow tasks are executed , then simply setting all the precedence constaint link b/w DFT 's to On Success(Green color) will work - http://msdn.microsoft.com/en-us/library/ms141261.aspx

or 

if you care only about the execution of one DataFlow task and just want the completion of other DFT' s (irrespective of successful / failed execution result ) , then setting ForceExecutionResult  to success on other DFT 's will work - http://sathyadb.blogspot.in/2013/02/sql-server-integration-services.html