In this paper, we evaluate the impact of signal to noise ratio (SNR) on the PHY-level packet loss rates on IEEE 802.15.4 links under the additive white Gaussian noise and Rayleigh fading models. We show that IEEE 802.15.4 PHY-level packet loss rate has a step-like response to the SNR deterioration. In other words, the packet loss rate is largely unaffected by SNR deterioration as long as SNR is more than a threshold. However, even a small deterioration in SNR beyond this threshold causes the packet loss rate to approach1. This result implies that SNR may not serve as a fine-granularity metric to indicate the reliability of an IEEE 802.15.4 link.