Early detection of lipid-rich vulnerable plaque is important to prevent aortic atherosclerotic plaque rupture, which can cause cerebral and cardiac infarction. Nevertheless, evaluating such plaques using conventional modalities is difficult. Photoacoustic imaging can reveal and clarify tissue characteristics. Some evaluations of lipid-rich plaque have used intravascular photoacoustic imaging systems. To reduce invasiveness and to ease handling, we developed a handheld photoacoustic imaging system. The possibility of detecting lipid-rich plaques was evaluated in phantom experiments. A plaque was modeled using a mimic plaque (a mix of oleic acid cholesterol and linoleic acid cholesterol) into an ovine aortic wall and silicone tube. The ovine aorta and the silicone tube were then filled with ovine blood. These objectives were fixed in pellucid deaerated water or in deaerated water including intralipid suspension. Laser light was guided to the model phantom surface by an optical fiber bundle close to the linear ultrasound probe. The photoacoustic signal distribution was measured as photoacoustic images. The photoacoustic images, taken using wavelengths at which light absorbance of lipid is high, show strong photoacoustic signals from the fat boundary. At 1150-1300 nm wavelengths, similarity between photoacoustic spectra and the absorption spectrum of lipid were evaluated by calculating the correlation coefficient in photoacoustic images. Results show high correlation (more than 0.9) at the boundary between the fat and the vessel wall. These analyses demonstrate detection of lipid-rich plaque even if a highly absorbing object, e.g. blood, is in proximity to the lipid.