Hi Vikas,
The bond strength of F-F is high owing to the small size. Small size means less repulsive forces between the electron clouds surrounding the atoms. In Cl2 molecule these repulsions are very high also the 3p obitals have less bond strength when compared to that of 2p in F2 molecule.
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bond energy (in kJ/mol)  F-F:158 Cl-Cl: 244
Hence, in order of decreasing bond strength: Cl-Cl => => (F-F) =>
Fluorine is an anomaly.
Bond strength decreases from chlorine to iodine as down the group, the atomic size becomes larger and thus the valence electron orbitals become more diffused, causing the overlap of orbitals to become less effective. Therefore the halogen-halogen bond becomes weaker.
Fluorine is an exception due to its extremely small size. The F-F bond length is so short that the lone pairs of electrons on the fluorine atoms repel each other and weakens the F-F bond. 
I hope that answers your question.and u approve it