Aptamers, DNA or RNA single-stranded sequences, have different applications in biological investigations, such as apatsensors, due to their many advantages including high specificity and affinity, cost-effectiveness and easy synthesis. In this study, an aptasensor was designed based on the changes in the SPR spectra of gold nanoparticle, in order to detect carcinoembryonic antigen (CEA) cancer indicator as a marker for breast cancer. In the presence of aptamer, gold nanoparticles were stable, SPR spectrum of gold nanoparticle was unchanged after adding NaCl. However, in the presence of CEA as a cancer marker, aptamer binds to the target molecule and by adding NaCl consequently the SPR spectrum of gold nanoparticles is changed. The results of this study showed that the designed aptasensor enables the detection of CEA over a range of 50 ng ml^-1. The limit of detection was about 22.75 ng ml^-1. It seems this aptasensor can be used in detection of carcinoembryonic antigen cancer marker.