A miniaturized embedded spiral-shaped microstrip antenna simulated within a block of 2/3 human muscle phantom is designed for implanted cardiac pacemaker. The new design not only achieves the good matching at the required 403 MHz but also obtains 57% size reduction over the existing design. The reflection coefficients are measured in both free space and in the human-tissue-equivalent dielectric liquid. Good agreement is achieved between simulated results and the experimental results.