Countries which don't have written constitution

 1.   Israel 

              Israel never adopted a constitution.  It has basic laws, court decisions and parliamentary statutes.

2.  United Kingdom 

              United Kingdom don't have written constitution.

3.  Saudi Arabia 

                   Saudi Arabia's governance primarily based on Quran and Sunni Islamic law.

4.   New Zealand 

                   Like UK, New Zealand don't have written constitution.

5.  San Marino

                   San Marino don't have single unified constitution. But it has parliamentary system and century old statutes.