Yes, henna straightens hair. You will get rid of curly hairs with use of henna, but slowly and steadily. It requires a lot of patience. After 5 applications you will see a little bit result. Actually, it depends on individual’s hair. If you have tight curls then henna will show no effects.
Here’s a bit more scientific explanation.
Hair comes in three basic types, straight, wavy, and curly. If you took a cross section of each individual strand of those you’d find this:
Straight hair: round
Wavy hair: oval
Curly hair: flat
Since henna coats the hair and binds with the kertain that makes up the hair strand, the coating rounds out the hair shaft a bit thus making it straighter. Some people hair is exceptionally curly and has a flatter strand that is less impacted by the henna coating. Someone who is more of a 2a who is just on the border of having some wave, the henna coating is enough to round out their strands and make them go straighter.