If I apply for China Visa can I go to visit Hong Kong also ?

Nary, you cannot only apply for one entry permit to visit both HK and mainland PRC because entry permits to HK and mainland PRC are separate. If you want to visit both places, you need to apply for two entry permits.

Thanks for the helpful answer. So what the documents does embassy need from me for China and Hong Kong Visa ? Thanks ! And how long I can get it ?

The documents for mainland PRC and HK are almost the same including your passport and copy of its data page, the application form, 3 recently-taken bareheaded photos and your detailed travel itinerary as well as financial support proof such as income certificate or work certificate, round trip air tickets and hotel reservation.
But for PRC: sometimes, the medical insurance in PRC may also be required by the consulate general.

If both applications are processed by the embassy of PRC in Cambodia, it usually takes 4 working days, while if the HK application is transferred to HK immigration department, it usually takes 4-6 weeks.