Roger Hargreaves' Mr Greedy is only slightly easier to read than John Steinbeck's classic Of Mice and Men, a study has found.
Mr Greedy, alongside Mr Men favourites Mr Tickle and Mr Happy, is also considered harder to read than Roald Dahl's Fantastic Mr Fox.
The books were rated by Renaissance UK, which analyses text complexity.
Jonathan Swift's Gulliver's Travels, the classic satire first published in 1726, was rated the hardest.
Renaissance UK examined more than 33,000 books for children and young people, scanning every page for sentence length, average word length and word difficulty level.
"Mr Men books look really easy but they are a really high difficulty level," said James Bell, director of professional services at Renaissance UK.
Of Mice and Men: opening lines
"A few miles south of Soledad, the Salinas River drops in close to the hillside bank and runs deep and green. The water is warm too, for it has slipped twinkling over the yellow sands in the sunlight before reaching the narrow pool. On one side of the river the golden foothill slopes curve up to the strong and rocky Gabilan Mountains, but on the valley side the water is lined with trees - willows fresh and green with every spring, carrying in their lower leaf junctures the debris of the winter's flooding; and sycamores with mottled, white, recumbent limbs and branches that arch over the pool."
Mr Greedy: opening lines
"Mr. Greedy liked to eat!
In fact, Mr. Greedy loved to eat, and the more he ate the fatter he became.
And the trouble was, the fatter he became the more hungry he became.
And the more hungry he became the more he ate.
And the more he ate the fatter he became.
And so it went on."
The research, which helps teachers find the right books for pupils, rates books on a scale of 0.2 (easiest) to 13.5 (highest), comes ahead of World Book Day on Thursday 7 March.
Mr Greedy was rated at 4.4 while Steinbeck's 1937 novella Of Mice and Men was rated 4.6.
The American author's 1939 Pulitzer Prize winning novel The Grapes of Wrath was rated marginally higher at 4.9.
Meanwhile, Roald Dahl's The Magic Finger and Fantastic Mr Fox, were rated at 3.1 and 4.1 respectively.
Dahl's The Twits was set on a par with Mr Greedy at 4.4, while Charlie and The Chocolate Factory scored slightly higher with 4.8.
Mr Bell said that sometimes teachers needed to be steered away from giving Steinbeck's texts to primary school pupils.
He said: "Of Mice and Men has a really low reading ability but the text is complex and of an adult nature.
"In theory, you could give a 4.6 book to a Year 6 child but you wouldn't want to give them it because it has content about adult themes and so on."
Top 10 'easiest' books
- Bad Bat by Laura Hambleton
- No, Sid, No! by Kate Scott
- The Go-Kart by Roderick Hunt
- The Magic Egg by Vivian French
- Sam and the Nut by Sheryl Webster
- Bess and Tess by Susan Blackaby
- Best Bird by Laura Hambleton
- Cat and Dog in a Mess by Shoo Rayner
- Get Fit by Gina Nuttall
- Got It! By Charlotte Guillain
Top 10 'hardest' books
- Gulliver's Travels (Unabridged) by Jonathan Swift
- Don Quixote (Unabridged) by Miguel de Cervantes Saavedra
- Ivanhoe by Walter Scott
- The Legend of Sleepy Hollow and Other Stories by Washington Irving
- Romeo and Juliet by William Shakespeare
- Vanity Fair by William M. Thackeray
- Robinson Crusoe by Daniel Defoe
- Frankenstein by Mary Shelley
- Mansfield Park by Jane Austen
- The Charterhouse of Parma by Stendhal