Coherency of the Al3Sc precipitates in Al-0.2Sc, Al-3.0Mg-0.2Sc and Al-2.5Cu-0.23Sc alloys was investigated by the quantitative analysis of spherically symmetrical strain contrasts around the precipitates in TEM images. Measurement of coherency strain was made utilizing the equations developed by Ashby and Brown, assuming negligible solutions of Mg and Cu in the Al3Sc phase. Elastic strain at the Al/Al3Sc interface in the binary alloy is nearly independent of the particle size. Those strains tend to be smaller in the Al-Mg-Sc alloy and be slightly larger in the Al-Cu-Sc alloy. The critical size of the precipitates for coherency-incoherency transformation becomes larger by the addition of Mg.