We propose isochronous-MAC (I-MAC) using the long-wave standard time code, and introduce cross-layer design for a low-power wireless sensor node with I-MAC. I-MAC has a periodic wakeup time synchronized with the actual time, and thus requires a precise timer. However, a frequency of a crystal oscillator varies along with temperature, from node to node. We utilize a time correction algorithm to shorten the time difference among nodes. Thereby, the preamble length in I-MAC can be minimized, which saves a communication power. For further power reduction, a low-power crystal oscillator is also proposed, as a physical-layer design. We implemented I-MAC on an off-the-shelf sensor node to estimate the power saving, and verified that I-MAC reduces 81% of the total power, compared to low power listening.