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Nancy Grace is showing no mercy to Lindsay Lohan ... and thinks the 25-year-old starlet should be in jail!

While in between dance rehearsals for Dancing With The Stars, the former prosecutor gave her opinion on Judge Sautner's sentence ... which has Lilo serving 30 days in jail once she surrenders by November 9.

Grace tells our X17 videographer:

    "I think that just on revocation, 30 days is good, but I think she should have gotten about a 2 year sentence a long time ago."

Hear Nancy's harsh words about whether Lilo will complete her probation ... in the video above.