Cl₂ is more reactive than l₂  but when KCIO₃ reacts with I₂ ,CI₂ is liberated.. why? 

Reaction of chlorate with iodine is a redox reaction and I₂ is oxidised.

halogen in positive oxidation state readily gains electrons and undergoes reduction and SRP of ${\mathrm{ClO}}_3^{-}/{\mathrm{Cl}}_2$ is greater than ${\mathrm{IO}}_3^{-}/{\mathrm{I}}_2$. Thus ${\mathrm{ClO}}_3^{-}$ readily under goes reduction while I₂ undergoes oxidation

$2{\mathrm{KClO}}_3+{\mathrm{I}}_2⟶2\mathrm{K}{\mathrm{IO}}_3+{\mathrm{Cl}}_2$