Which Season & Episode Did Clark Tell Lana About His Secret?

Lana finds out about Clark's secret before he tells her in season 6 episode 16 called "Promises". This is the episode where Lana has second thoughts about marrying Lex and decides to go and find out the truth about Clark.

Once she has found out the truth through deception, she promises Clark that she won't marry Lex. But, Lionel, having found out what Lana is going to do, forces her to go down the aisle to marry him. At this point Clark is unaware that Lana knows the truth about him but is shocked when he hears that Lana married Lex.

• Smallville

Smallville is an American television series which is meant to be a prequel to Superman. The main character is a younger Clark Kent (played by Tom Welling) who has super human powers and tries to find his place in life after finding out how abnormal he is. The show is based on the DC comic, Superman. The show was originally aired by the WB on 16th October 2001 and ended on its tenth and final season in May 2011.

• Ratings and records
  

Over the ten seasons it was aired it received an average of 4.34 million viewers per episode. By the end of the show it had become the longest running comic book based series as well as the longest-running North American science fiction series in television history.

Season 6 - "Phantom".
Air date: Thursday May 17, 2007
http://www.tv.com/smallville/phantom/episode/1001723/recap.html