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How to Find Bapi for Particular Transaction in SAP

How to find function module or Bapi for particular transaction in sap?

If you mean that you need to know what BAPI's a particular tranx uses, which I can only assume that's what you mean, then you should access the code behind the transaction and search for 'CALL'. That normally is the standard method that think that most people use. 

Suppose you want to find the bapi for creating a sales order, you usually use transaction VA01 for this. 

1. Find out the package of the transaction.  

Start Va01 go to system --> status.  

Double click on transaction  

Package is VA  

Open this package in SE80  

Open business engineering-->Business object types  

Find the BO which sounds the most appropriate  

I would suggest BUS2032 Sales Order  

Double click.  

Open methods.  

Find the released method with from data or something similar in the name  

, Createfromdat2  

Position the cursor in it and click the program button  

Scroll down to find the bapi used in this method 

With this way you can also find out programs and FM's 

2. Start va01 go to system-->status  

Double click transaction VA01  

Double click on package  

Read the application component. (this is SD-SLS Sales)  

Then open the transaction BAPI  

Sales and distribution-->Sales-->sales order  

createfromdat2