Can any body tell me how to add fund from a merchant bank account to paypal using paypal api ?

for example I have a bank account and i want to upload some fund to
paypal How to do it using api ?

When you have a merchant account they dont hold your money instead the money goes directly into your checking account so I am not sure how you would do that being that the merchant account does not hold your money like a paypal account does. Most merchant accounts are only for accepting cc payments or accepting echecks and not for paying with.
What you would have to do is to transfer your money from your bank account to your paypal account;

To add funds from your bank account to your PayPal account, you will first need to confirm your bank account. Confirming your bank account will set up bank funding.

Once your bank account has been confirmed, you can follow these steps to add funds:
Log in to your PayPal account
Click ‘Add Funds’ at the top of the page
Click ‘Transfer Funds from a Bank Account’
Enter the amount you wish to add
Click ‘Continue’
Review the details and click ‘Submit’
Click ‘View the details of this transaction’ to see the expected clearing date for the funds transfer
The funds will become available in your PayPal account balance in approximately five to seven working days.

You can view the current status of your bank transfer by going to your PayPal transaction 'History' and checking the transaction status:
Pending: you requested an electronic funds transfer
Processing: your request for funds has been sent to the bank
Completed: the funds transfer to your PayPal account has been completed and the funds are in your PayPal account balance
Refused: your funds transfer was rejected by the bank

PayPal is normally implemented on a site to accept payments, which adds funds to your account if the payment is made. Adding funds to your account directly is usually done from your bank account so you can use those funds to make a purchase. That isn't something that you would normally use an API to do (and I'm not sure if the API actually supports that)

If you want to be considered as a serious site you should first offer people to pay via VISA master Discovery and Amex. Get a merchant account and than you can add if you wish Paypal or Google

My 2 cents,

Nuriel