Hi, All. I want to know where I can disable/enable creating PO automatically when SO created

Depends on the item. On the item record directly you can turn on drop ship and then define a preferred vendor on that item record. Then when you ring up the item in a sales order a PO will be created automatically. Note: Drop ship will have the PO ship to directly to end customer. If you want the PO shipped to you first you can check the special order box on the item record

Thanks for your help. This is what I want to do. When SO created, PO created automatically for dropship. But when PO created, it sends email to vendor automatically. But I want to disable it for specific vendor. How can I do it?

Never done that for specific vendor- logically thinking you may need scripting to achieve that, not sure workflow is capable of doing so

Thanks, how can I do it using script. I have to use UserEvent script?

Turn off the "Send Transactions Via" email checkbox on the vendor record, and I believe that will resolve your issue. This checkbox controls the "To be emailed" checkbox on the PO which sends or doesn't send an email.