require in environment.rb is failing in unit test

When I run unit test it fails.

/home/gobi/workspace/0001_myweb/config/environment.rb:1:in `require': no such file to load -- action_mailer (LoadError)         from /home/gobi/workspace/0001_myweb/config/environment.rb:1         from ./test/unit/../test_helper.rb:2:in `require'         from ./test/unit/../test_helper.rb:2         from test/unit/user_test.rb:1:in `require'         from test/unit/user_test.rb:1

But when I run rails application it is working no problem.

How can I run the Unit Test.

Could someone teach me how?