The present invention provides co-crystals of Aciclovir that has a improved solubility compared to pure Aciclovir wherein the said co-crystals axe prepared with co-formers preferably Tartaric Acid, Succinic Acid, Malonic Acid, Glutaric Acid and Adipic Acid. The ratio of co-formers optimized for the said co-crystals is 1:6. The process for preparation of said co-crystals uses environmentally safe solvent i.e. water.