The present invention relates to a novel peptide analog of acylated oxyntomodulin and a pharmaceutical composition comprising the same for preventing and treating obesity or overweightness, or diabetes accompanied by obesity and overweightness. The peptides are superior to those of natural oxyntomodulin in dual agonism on GLP-1 and glucagon receptors and longer in vivo half-life. A pharmaceutical composition comprising said peptides is effective in the treatment of metabolic diseases such as obesity and diabetes mellitus.