Which one has a higher boiling point, hexane or hex-1-ene? Why?

In fact, I think hexane has a higher boiling point because it has 2 more hydrogen atoms than hex-1-ene, so it has a larger molecular size, i.e. stronger intermolecular force between the molecules.
But, is it correct if I say so?

That's a good deduction.  I believe you are correct with this.  However, I think it's more due to the structure of the molecules than their mass.  The hex-1-ene molecule has a little bit of a 'kink' in it, so it can't get as close to it's brethen as hexane can.  As a result, the intermolecular attraction is less.  This causes its boiling point to be lower.  Hexane is a pretty linear molecule, so the individual hexane molecules can get closer to each other and exert a greater attraction.  As a result, its boiling point would be higher.