Lead-free Sr2K0.1Na0.9Nb5−xTaxO15 ceramics were prepared using the conventional mixed-oxide method. The phase transition corresponding to the tetragonal ferroelectric (4mm) to paraelectric (4/mmm) phase was observed with increasing x according to the XRD and dielectric properties results. Ta5+ substitution for Nb5+ inhibited the grain growth and also prevented the abnormal grain growth behavior. The dielectric and ferroelectric properties of the ceramics decreased and even vanished with increasing Ta5+ substitution. Besides, the underlying mechanism for variations of the structures and properties due to Ta5+ substitution was explained in this work.