Flame spread over thin electrical wire under different oxygen concentration has been investigated both theoretically and experimentally to gain a better understanding of electrical fires in various atmospheric environments. A simplified flame-spread model is developed to identify the effects of wire configuration, thermal conductivity and oxygen concentration on spread rate. Two kinds of sample wires, made of nickel-chrome and copper as the metal core, with different configurations are adopted in this study. The environmental oxygen concentration is changed from 21% to 80% under two cases: A fixed total pressure of 1 atm and a fixed oxygen partial pressure of 21 kPa. Experimental results show that in either case the flame spread rate monotonically increases with oxygen concentration. The difference of spread rates between these two wires depends on both the wire configuration and thermal conductivity. For the wire with the smaller diameter and higher thermal conductance, the flame spread rate is large. As the oxygen concentration increases, the flame becomes brighter and wider and the dripping condition of molten polymer will also change. Also, the comparison of two cases shows that the spread varies sightly with the environmental pressure changes. The experimental results agree well with the model predictions.