我想問下網上面講粗口犯法嗎?!

At internet, Generally it is not. But very old common law case may indicate an offence. called intent to corrupt public morals. Very very old principle and vague enough for the prosecution to make anything into an offence.

Orally in public ...
At airport, MTR station, it is an offence under the laws of Hong Kong. Public area even to the public, not an offence. But if you use bad language against an public officer who is performing their duties, still liable. These officer include police, fireman ....

當然犯法，係街講粗口係犯法，何況是在MSN 戈D講粗口，人哋會告你。