The relationship between serum Mg and blood cell counts in Chinese adult diabetes or central obesity was assessed by investigating 8163 subjects with China Health and Nutrition Survey (mean age 59⋅6 years, 54⋅9 % men). Participants were classified according to blood Mg (below 0⋅65 mmol/l, or 0⋅66–0⋅94 mmol/l or above 0⋅95 mmol/l), type 2 diabetes (yes/no) and central obesity (yes/no). Leucocytes, erythrocytes, platelets (PLT), Hb and glycated Hb (HbA1c) were determined using standardised methods and conditions. HbAc1, leucocytes and PLT were significantly higher among subjects with central obesity than without central obesity (P < 0⋅05). A significant increase for Hb, erythrocytes, PLT, but not leucocytes, across progressive Mg groups was observed in subjects without diabetes (P < 0⋅05). Hb, erythrocytes and HbAc1 were significantly higher among subjects with higher Mg than in subjects with lower Mg with diabetes (P < 0⋅05). Central obesity disturbed the positive association between PLT count and serum Mg. Type 2 diabetes caused metabolism disorder in serum Mg, blood sugar and blood cell count. Hb, erythrocytes and PLT, but not leucocytes, are positively correlated with serum Mg, but this association is somehow disturbed by type 2 diabetes or central obesity.