The optical microscopy, crystallinity and ion conductivity of PEO complexed with potassium iodide (KI) salt are present. The spherulite structure derived from polarized optical microscopy (POM) suggested the ion induces different spherulite structures from that without salt. The size of the spherulites decreases with increasing salt concentration and is completely destroyed with KI salt content of 20 wt% (O–K ratio 15:1). This result concurs with the X-ray diffraction and DSC studies, that PEO crystallinity is reduced upon addition of KI salt. Upon elevating the temperature, the POM micrographs elucidate degradation of the spherulite structure when smaller crystallites begin to melt before the melting temperature of SPE, and become completely opaque above PEO melting temperature (T m). The pseudo-activation energy derived from variable temperature conductivity measurements of about 0.24 eV is similar to that of PEO–lithium salt systems and suggested the identical PEO segmental motion governs the fundamental ion movement. Stable PEO–K complex (T m from 135 to 155°C) is formed after annealing at 120°C for 12 h. However, the conductivity is about one order smaller compared to lithium salt due to the shorter hopping distance of the heavier potassium ion.