A former Federal MP has been found guilty of stealing $100,000 from a family trust.

The former Liberal member for Tangney, Peter Shack, denied the charge, claiming his mother-in-law agreed to lend him the money in 2004 for an investment.

He testified that his mother-in-law wanted to keep the loan a secret from her children to avoid any claims of favouritism.

But today a District Court jury in Perth found Mr Shack guilty after several hours of deliberations.

He is due to be sentenced in April.