Airline stocks are just too cheap right now for Jim Cramer.

In fact, the valuations of stocks such as American Airlines, United Continental, Southwest and Delta are downright insane.

"The airlines are a buy, plain and simple," the "Mad Money" host said. (Tweet This)

Delta earned $5.9 billion in 2015, dished out $2.6 billion in dividends and share repurchases and there are only 770 million shares outstanding. And while the stronger could ensure that passenger miles won't be as good this year, Cramer couldn't deny the huge impact that lower fuel costs could have to its bottom line.

Likewise, United made $4.8 billion last year, or $11.88 per share, which calculated to a $58 stock. And the company bought back $1.2 billion in stock and has $.1 billion in operating cash flow yet there is no doubt for Cramer that this stock is way too cheap right now at current levels.

American Airlines raked in $6.3 billion in 2015, which was a 50 percent increase year-over-year. It repurchased 85 million shares and has almost $7 billion in cash with a $25 billion market cap. Cramer also expects this airline to benefit significantly from the decline in oil process.

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"It's hard to beat those percentage increases from last year, which is part of the undervaluation. But you have to understand, this is an industry that has never earned back its cost of capital until now," Cramer said.

Without new competition and full plane capacity in most places — except Latin America, which has been hurt by the Zika virus — coupled with the strong dollar and strength of the industry, Cramer added that this is absurd.

What could hold the airlines back?

Cramer spoke with Southwest Airlines' co-founder and former-CEO Herb Kelleher on CNBC's "Squawk on the Street" on Monday. Kelleher shared that he is worried that the government could be getting involved again via stricter regulation.

Yet airline stocks remain a screaming buy in Cramer's perspective.

"I can't stress it enough; neither could Herb Kelleher. And who could ever disagree with him?" Cramer asked.

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