1. Handwritten IOUs have legal effect, but they must meet the following conditions:

(1) must be something that can prove the fact of debt;

(2) It must be the expression of the true meaning of the parties;

(3) The receipt of IOUs must be legal;

(4) Both parties must have full capacity for civil conduct;

(5) The content of the loan is legal.

2. Legal basis: Article 667 of the Civil Code of People's Republic of China (PRC).

A loan contract is a contract in which the borrower borrows money from the lender, repays the loan at maturity and pays interest.

Article 668

A loan contract 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.

Second, how to write an IOU can take effect in law.

1. The legal full names of the borrower and the lender shall be clearly written;

2. The loan amount should be clearly written in Arabic numerals and Chinese capital at the same time;

3. The term of the loan should be clearly written, including the start and end dates of the loan and a clear term of the loan, preferably accurate to the day;

4. The specific repayment date should be clearly written, but it is not necessary;

5, should write clearly the interest of the loan, there should be a clear annual interest rate or monthly interest rate, and finally the total loan interest payable;

6. The date, month, day, time and payment method of repayment of loan principal and interest shall be stated;

7. The borrower shall have his own signature, handprint or handwritten signature;

8. If the amount is large, you can ask a third party to witness and sign it.