Organic light-emitting diodes (OLEDs) have demonstrated remarkable advancements in both device lifetime and luminous efficiency. However, insufficient operation lifetime due to device degradation remains a major hurdle, especially for brighter devices. Understanding the degradation mechanisms of OLEDs due to the degradation of functional materials and the formation of defects in device architectures continues to be a significant challenge. Herein, we evaluate the degradation characteristics by scrutinizing the electrical and optical properties, as well as analyzing the charge carrier dynamics in pristine and aged states of phosphorescent OLEDs (PhOLEDs). We show that degradation mechanisms in PhOLEDs can be elucidated in terms of the ideality factors of current and luminance in pristine and aged device states. The consistent shifts in distinct ideality factors across various states points out that the device degradation is attributed to the deterioration of the guest material, i.e. green-light-emitting phosphorescent material. Conversely, the incongruity in ideality factor changes between the two states indicates that the degradation results from the deterioration of non-light-emitting material. Subsequent characterization experiments provide further evidence that this degradation is primarily attributed to the deterioration of CBP-host material. The thorough understanding of degradation mechanisms established in this study can contribute to realizing the highly reliable PhOLEDs with a long lifetime.