A brute force algorithm should be evident for you. However, you didn't specify any requirements on the algorithm.

@Osinenko: Thanks you for your suggestion. Actually, my project is people counting system using stereo camera. My task is to find the HEAD. Therefore, I think I should find the local minimum areas in the image. Refer to the sample image.

Then, you should search in direction of image recognition. NNs are a particular solution used in this field.

Finding people's heads via a local minimum? What about straw hats and blondes??? Or baldheads?

You'd better concentrate on 'near-to-round' objects amidst some ovoid (the shoulders). Still to take care of bellies and other 'objects' :)

try "imregionalmin" function in matlab

 @Aparna Murthy, @Abdullah H. Ozcan: Thanks for your suggestion. I will try them.