Provided are a catalyst composition including a transition metal catalyst and a nitrogen-containing bidentate phosphorus compound and a process for hydroformylation reaction of olefins to prepare aldehydes which includes stirring the catalyst composition, an olefin compound, and a gas mixture of of carbon monoxide and hydrogen, under high temperature and pressure condition. Therefore, very high catalytic activity and high selectivity in normal-aldehyde or iso-aldehyde according to the type of a substiuent are ensured.