Terrain
BMX bikes and mountain bikes are constructed to ride on different types of terrain. BMX bikes are made to ride on streets, halfpipes and dirt jumping courses, whereas mountain bikes are made to withstand tougher terrain such as steep hills, rocks and mud.
Wheels
BMX bikes have a standard 20 inch wheel, whereas mountain bikes typically have wider tires due to the difficulty of terrain ridden. However, both bikes require good traction on their tires to ensure stability and balance.
Frames
BMX bikes have smaller than normal frames to help riders maintain control in the air and when landing tricks. Mountain bikes have heavy, sturdy frames to increase stability on rough terrain.
Handlebars
BMX bikes have unique handlebars in that they can spin 360 degrees around enabling riders to add difficulty to their tricks. On the other hand, mountain bikes have wide handlebars that are perpendicular to the ground giving the rider greater control.
Gear/Suspension
BMX bikes typically only have one gear (however some have two), whereas mountain bikes usually have 20 to 24 gears. Also, BMX bikes lack the complex suspension of mountain bikes while mountain bikes are classified by their type of suspension.
