Study on the diffusion growth of ternary intermetallic compounds in Mg-Al-Zn based light-weight alloys is important due to its close interrelation with alloy property. However, there is a very lack of existing data due to difficulties in both experimental and computational aspects. The current work aims at presenting the experimental observation on the diffusion growth behavior ofφphase at 360℃ as well as calculating its composition-dependent interdiffusion coefficients. We designed and successfully fabricated four Mg-τternary diffusion couples annealed at 360 ℃ for different times, where the diffusion path goes across theφphase region and the diffusion growth of ternary intermetallic compound can be solely detected. In-situ observation of the time-dependent growth ofφphase was performed to accurately determine the parabolic growth constant. The experimental data were then subjected to a numerical inverse method to generate a set of self-consistent interdiffusivities of the ternary intermetallic compounds, which can reproduce the presently observed diffusion growth behavior of φ ternary intermetallic compound in Mg-τdiffusion couples.