Freedom of contract is a fundamental principle in contract law. However, whether a clause in the contract restricting the form of the contract, especially for the variation of the contract, being a voluntary restriction on the freedom of contract, is legally effective, has been highly controversial. This paper discusses the competing policy concerns for certainty versus freedom of contract and the rationale underpinning them. It critically scrutinises the relevant statutory provisions and case authorities in some civil law, common law jurisdictions and in international treaties or practices; and make recommendations accordingly for the improvement of the relevant existing contract law rules, and Chinese contract law rules in particular.