And the Government will ask the Environmental Protection Authority (EPA) to reassess its proposal for the controversial highway.
Roe 8 will link Roe Highway to Stock Road through wetlands in Bibra Lake.
In December, the WA Supreme Court ruled conditional approval granted by the EPA for Roe 8 was invalid.
Premier Colin Barnett said the decision would delay the PFL but not end it. On Friday he confirmed his Government would appeal the decision.
“An appeal is necessary to provide clarity for assessment and approval processes undertaken by State agencies and local governments,” he said.
“The decision to appeal is based on advice from the State Solicitor’s Office that it considers there are valid grounds to support an appeal.
“The Supreme Court’s decision is likely to have implications more broadly for government decision-making.
“The decision’s importance extends beyond just Roe 8; it is an important one for all of government because it deals with the question of when a government policy becomes a mandatory relevant consideration for decision-making.”
Environment Minister Albert Jacob said the EPA would reassess its Roe 8 proposal.
“The EPA will decide how the assessment will proceed,” Mr Jacob said.
Opposition transport spokeswoman Rita Saffioti said the State Government had an “unhealthy obsession” with Roe 8.
“The Government is making out that this project is the panacea for everything; that it will fix all ills across WA,” she said.
“They have made it into something that it’s not.”