In　situ　lossy　compression　allowing　user-controlled　data　loss　can　significantly　reduce　the　I/O　burden.　For　large-scale　N-body　simulations　where　only　one　snapshot　can　be　compressed　at　a　time,　the　lossy　compression　ratio　is　very　limited　because　of　the　fairly　low　spatial　coherence　of　the　particle　data.　In　this　work,　we　assess　the　state-of-the-art　single-snapshot　lossy　compression　techniques　of　two　common　N-body　simulation　models:　cosmology　and　molecular　dynamics.　We　design　a　series　of　novel　optimization　techniques　based　on　the　two　representative　real-world　N-body　simulation　codes.　For　molecular　dynamics　simulation,　we　propose　three　compression　modes　(i.e.,　best　speed,　best　tradeoff,　best　compression　mode)　that　can　refine　the　tradeoff　between　the　compression　rate　(a.k.a.,　speed/throughput)　and　ratio.　For　cosmology　simulation,　we　identify　that　our　improved　SZ　is　the　best　lossy　compressor　with　respect　to　both　compression　ratio　and　rate.　Its　compression　ratio　is　higher　than　the　second-best　compressor　by　11%　with　comparable　compression　rate.　Experiments　with　up　to　1024　cores　on　the　Blues　supercomputer　at　Argonne　show　that　our　proposed　lossy　compression　method　can　reduce　I/O　time　by　80%　compared　with　writing　data　directly　to　a　parallel　file　system　and　outperforms　the　second-best　solution　by　60%.　Moreover,　our　proposed　lossy　compression　methods　have　the　best　rate-distortion　with　reasonable　compression　errors　on　the　tested　N-body　simulation　data　compared　with　state-of-the-art　compressors.　©　2017　IEEE.