Based　on　systematic　analysis　on　rock　magnetism　and　grain　size,　this　paper　studies　the　topsoils　of　the　loess　region　of　Tianshan　Mountains　and　the　Loess　Plateau　in　northwestern　China.　Their　magnetic　characteristics　indicate　that　the　main　magnetic　minerals　of　these　topsoils　are　magnetite,　maghemite,　hematite　and　lepidocrocite　or　goethite.　The　grain　size　of　magnetic　minerals　in　Tianshan　topsoils　is　generally　much　coarser;　mostly　within　range　of　multi-domain　(MD).　Hard　magnetic　minerals　in　Tianshan　topsoils　are　higher　than　that　of　Loess　Plateau.　The　magnetic　susceptibility　of　topsoils　from　Loess　Plateau　shows　a　good　positive　relation　with　frequency　dependent　susceptibility.　Climate　condition,　such　as　temperature　and　precipitation,　especially　the　latter　one,　is　considered　as　the　major　factor　effecting　susceptibility　enhancement　in　the　Loess　Plateau.　Tianshan　topsoil　contains　a　very　small　amount　of　ultrafine　magnetic　grains　produced　during　pedogenesis,　which　has　very　limited　contribution　to　the　susceptibility　enhancement.　Tianshan　topsoils　generally　show　a　characteristic　of　less　weathering　or　weak　pedogenesis,　aeolian　coarser　grains,　therefore　play　an　important　role　in　mechanism　of　magnetic　susceptibility　enhancement.　Frequency　dependent　susceptibility　shows　a　close　relation　with　mean　annual　precipitation　(MAP)　in　all　studied　areas.　However　magnetic　susceptibility　demonstrates　difference　with　MAP　in　various　regions:　showing　positive　correlation　in　the　central　Loess　Plateau　(studied　area);　but　negative　relation　with　MAP　in　Tianshan　area.　Therefore,　great　care　should　be　taken　when　explaining　environments　using　magnetic　susceptibility　values　from　different　regions.