A formal IOU should have the following contents: 1, lender's name and ID number; 2. Indicate whether to lend in cash or by transfer; 3. Write down the loan amount. The amount should be written in lowercase and uppercase to prevent changes; 4. Indicate the loan term; 5. Write down the loan interest rate; 6. Ask the borrower to write the ID number according to the fingerprint; 7. Indicate the date of loan. An IOU is a written document indicating the relationship between creditor's rights and debts, which is generally written and signed by the debtor, indicating that the debtor has owed the creditor the amount indicated in the IOU. Article 668 of the Civil Code of People's Republic of China (PRC) shall be in written form, unless otherwise agreed between natural persons. The contents of a loan contract generally include terms such as loan type, currency, purpose, amount, interest rate, term and repayment method.