Abstract: Single word multiple upsets(SMU) are studied in two kinds of CMOS static RAM by F, Cl and Br ions from HI 13 tandem accelerator. To distinguish SMU from all single event upsets (SEUs) detected, high resolution test system using network technique has been developed. SMU σ LET curves of different test patterns 5A, FF and 00 are presented and the effects of LET on the results are analyzed. Experimental results have important implications on space upset rate prediction.