  We propose a systematic procedure to work out systems of topological defect
lines (TDLs) in minimal models. The only input of this method is the modular
invariant partition function. For diagonal and permutation diagonal models, we
prove there is a bijection between simple TDLs and primary fields preserving
fusion rules. For block-diagonal models, we work out simple TDLs in the
$3$-state Potts model as an example. The results agree with those in $3D$
topological field theory methods.
