If I have invitation letter from a Chinese company can I obtain a visa without an agent.

I assume that you need to apply for the M type one. M is issued to those who engage in trade activities in PRC. You have to provide your passport (with the validity of more than six months and blank pages), filled application form affixed with your recent photo (2X2 inches), visa notification form, an orginal guarantee letter from Chinese company, an introduction letter from your employer, both original and copies of your ID card, both original and copies of your company business license, hotel booking and round-trip air tickets, recent financial statement, copies of your passport information page and entry records for PRC and other documents required by the embassy.