This research studies of crushed oyster shell (OSP) as a substitution for cement in producing the interlocking block. Research process are design the mixtures which contain 0 10 20 30 40 and 50%. Control water at optimum moisture content (OMC). Results of the tests the compressive strength at 7 14 28 60 90 days and the moisture absorbed at 28 days showed that increasing the amount of OSP decreased compressive strength and increased water absorption. By using OSP to replace cement 10% 20% passed the community product standard 602/2547 these were classified as non-load supportive type