How can I change the defult date format in oracle10g enterprise?

Question: I use alter session & NLS_DATE_FORMAT, but it's not working.

The ALTER SESSION in Oracle SQL only affects the current session (as the name implies). This means that anybody else connected to the database won't be affected and neither will any future sessions - so if you log off and log on again, the NLS_DATE_FORMAT reverts to the default (which is set by database initialisation parameters).

Therefore to change the date format for everyone you will need to change the NLS_DATE_FORMAT parameter in the Oracle database initialisation file.

Oracle documentation covers globalization settings quite comprehensively in the Oracle database installation guide but settings can be changed after installation (as noted above) by changing the values of parameters.