We are now developing a multi-slice depth-of interaction (DOI) MR-compatible PET system (K-PET I). The PET is for imaging small animals such as mouse or rat. It employs DOI block detectors made of MLSs, optical fibers, and multi-channel position sensitive photo-multiplier tubes (PSPMTs). The size of a single MLS was 2.5mm x 3.5mm x 3.5mm and eight MLSs were combined to form a DOI block detector proposed by Murayama  and connected to four optical fibers via light guides. Thirty two sets of block detectors were arranged in a circular ring of which diameter is 51mm. Position histograms showed good separations for all block detectors. Timing resolution of 6ns FWHM was obtained with the compact PCI based coincidence system. The PET has 3 imaging slice, two layers DOI detection capability, and spatial resolutions of around 2mm FWHM in transaxial and axial directions. The K-PET I will be combined with an animal MRI system for molecular imaging research using small animals.