Question: Please tell me that what sholud a "PURCHES ORDER " letter should consider ? 

Answer: 1. Name, address, phone number of your company (both shipping and billing addresses, if different)\n2. Name, address, phone number of the vendor\n3. Date of Order\n4. The quantity of the item you want to purchase\n5. The description and serial/catalog number of the item\n6. The agreed price of the item + any applicable taxes\n7. Term of sales.  Payment due date and/or discount info\n8. Shipping terms. \n\nThese are the key points.  But don't reinvent the wheel, there are many software or books on Purchase Order or inventory management.  I am sure there are lots of examples of what a PO looks like.  Good luck.
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