A cross sectional study on the association of Blood Lead Levels (BLL) and Working Memory (WMICS) ability of primary school children surrounding ex-copper mining area in Ranau, Sabah was conducted from September 2012 till October 2013. One hundred schools children were selected from 5 schools which consisted of three from Ranau (expose groups, n=60), one each from Pitas and Sipitang (non-expose groups, n=40). The main objective is to study the association of BLL and WMICS ability of Primary School Children in Ranau Sabah. Blood samples were taken by venous blood draw using disposable syringes and collected in plastic sterile tube mixed with K2EDTA. The samples were maintained at 4ºC and transported to University Malaysia Sabah laboratory. BLL was tested using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) Perkin-Elmer. WMISC ability was measured by extraction of McCarthy Intelligent Quotient (IQ) Index Test and known as WM Index Cumulative Score (WMICS). Demographic background obtained from the distributed questionnaires. Results showed the BLL range between 0.90 – 9.50µg/dL and significantly lower compare to BLL action level at 10µg/dL. The mean of BLL showed significantly higher in boys compared to girls (t=4.233; p<0.01). There was no significant difference of BLL between the expose and control groups (t=1.395; p>0.05). Ultimately result showed there was an inverse association for BLL and WMICS (r= -0.621; p<0.01) even though at the low BLL. In conclusion, the BLL as low as 4.96µg/dL still have negative association with WMICS for the primary school children surrounding ex-copper mining area in Ranau, Sabah. Lead elimination programme are essentially need to be implemented in study area. Further research is needed such as to carry out a comprehensive and complete lead study and monitoring programme which involves all parameters, such as environmental samples, biomarkers, nutritional factors and complete IQ test together with continuous health promotion and education programmes with inter-agency ...