#1
I have a Behringer HD300 Heavy Distortion connected with my Fender Stratocaster guitar.

Guitar -> Effect Pedal -> Amplifier

When I turn off the effect pedal, sounds okay, clean. But when I turn on the effect. So loud humming I hear, there is nothing I can hear but humming only, no distorted sound from my pedal. But when I turn it back to off, theres clean sound from my guitar.

Am I using a bad cable?
#2
So no sound come out of the pedal besides the hum when it's on?
Was it working before?
If so, it unlikely that it's a cable issue, the pedal is probably broken. Either one of the components got fried or something might have got loose or developed a cold solder joint. If you have some basic soldering skills and some electronics knowledge you may open it up and try to fix it, if you don't you might just get another pedal as a professional repair cost may be higher than the cost of the pedal itself.
#3
There is a little sound (distorted sound) coming from the pedal when its on, but it is likely dominated by the humming sound.
#4
I doubt the problem is caused by your cables, considering that it sounds fine when the pedal is turned off.

Is the pedal powered by batteries or power supply?

If you are using a power supply, try it with batteries. If the hum is still there, there's most likely something wrong with the pedal. Behringer is a cheap brand that's really not known for its reliability so I would really not be surprised.
Quote by AlanHB
Just remember that there are no boring scales, just boring players.

Gear

Bach Stradivarius 37G
Charvel So Cal
Fender Dimension Bass
Hartke HyDrive 210c
Ibanez BL70
Laney VC30
Tokai TB48
Yamaha FG720S-12
Yamaha P115
#6
the issue may well be electrical in nature. if where you have it plugged in produces a lot of 60 cycle hum that may be an issue when using a strat (or other single coil pickups.) also check the ground on your guitar as once again that could be the problem. high gain really brings these kind of issues out.
#7
Oh, I figure it out that the humming still there even the cord is not connected to my guitar and the pedal is on.
#8
Is there any hum if you plug your guitar straight to the amp? What if you use your amp's distortion? Try this with both of your cables.
Quote by AlanHB
Just remember that there are no boring scales, just boring players.

Gear

Bach Stradivarius 37G
Charvel So Cal
Fender Dimension Bass
Hartke HyDrive 210c
Ibanez BL70
Laney VC30
Tokai TB48
Yamaha FG720S-12
Yamaha P115
#9
It is working fine. Oh my, I think I should give up my pedal. Goodness, I just bought it recently, about 2 days ago.