Partial Volume Effect (PVE), the image blurring caused by the finite spatial resolution of PET imaging system, can cause incorrect quantification of radioactivity especially for small objects. High-resolution small animal PET system has been developed for rodents, but sizes of rodent hearts are still close to the spatial resolution, that might cause significant PVE than human heart in clinic PET. The aim is to evaluate PVE of various heart-shaped phantoms in a small animal PET system utilizing Monte Carlo (MC) simulation. We built a simulation platform using a MC simulation (SimSET+GATE) and reconstruction software (STIR) based on the scanner configuration of a small animal PET system (Inveon). To confirm the spatial resolution, a line source in a water-filled cylinder was measured and simulated at the center of field of view. Simplified model of heart, combinations of thickness (0.5-18mm) and inner diameter (1-10mm) of hollow-cylindrical source, were simulated with F-18 and Ga-68. Sinograms of true events were rebinned by FORE and reconstructed by 2D OS-EM (32 updates) with attenuation correction and Gaussian post-filtering (1.4mm FWHM). Recovery coefficients (RCs) were estimated by average over 50% of maximum value using a big cylindrical source (40mm diameter) as the reference. RC at the specific size was estimated by bilinear interpolation. Averaged FWHMs on the sinogram for F-18 and Ga-68 were about 1.7 and 3.9mm in MC simulation and 1.8 and 3.7mm in real measurement, respectively. RCs were 30.4% and 21.2% for F-18 and 20.5% and 14.5% for Ga-68 at the end of diastolic phase (ED) of the left ventricle (LV) of a rat and a mouse heart size (measured on MRI; 8 and 1 mm, 4 and 0.65mm for inner diameter and wall thickness, respectively). We developed a MC simulation platform and observed considerable count reductions (70-85%) by PVE in rodent heart size with small animal PET. This platform has the further potential use for developing new PVE compensation algorithms.