In Harry Potter and the Goblet of Fire, why didn't Harry Potter tell on Lucius Malfoy? I mean that he is in fact Death Eater.
He did, but the scene wasn't included in the movie.
After the imposter Moody, Barty Crouch Junior, was kissed (executed) by Dementors, the Minister is looking for Dumbledore and shows up in the infirmary where Harry is recovering from his injuries. At this point, Harry tries to tell him about Voldemort's return and lists all the Death Eaters who Voldemort named, but the Minister doesn't believe him:
"Look, I saw Voldemort come back!" Harry shouted. He tried to get out of bed again, but Mrs. Weasley forced him back. "I saw the Death Eaters! I can give you their names! Lucius Malfoy -"
Snape made a sudden movement, but as Harry looked at him, Snape's eyes flew back to Fudge.
"Malfoy was cleared!" said Fudge, visibly affronted. "A very old family - donations to excellent causes -"
"Macnair!" Harry continued.
"Also cleared! Now working for the Ministry!"
"Avery - Nott - Crabbe - Goyle -"
"You are merely repeating the names of those who were acquitted of being Death Eaters thirteen years ago!" said Fudge angrily. "You could have found those names in old reports of the trials! For heavens sake, Dumbledore - the boy was full of some crackpot story at the end of last year too - his tales are getting taller, and you're still swallowing them - the boy can talk to snakes. Dumbledore, and you still think he's trustworthy?"
- Harry Potter and the Goblet of Fire, Chapter 36: The Parting of the Ways