For men

--Regular shirts with collar and buttons. Polo shirts are not considered as regular shirts although they have collar and buttons.
--It is OK to wear a tie even on business casual day. If you are in Sales and you need to see customers, a shirt (long or short sleeves) with a tie is very appropriate for everyday.
--Shirts may be white or in color. In case of color shirts, the color should be light, such as light blue or beige.
It is OK to have color pattern such as stripes (条字) or check (格字) as long as they are thin and small. Bold stripes and large checks are inappropriate.
--Trousers may be in light or dark colors. Be careful when you choose light colors. Plain tones such as khaki (卡其) is OK. White, yellow, pink, etc are obviously not OK.
--Shirts and trousers must be ironed. Wrinkled shirts and/or trousers look very unprofessional.
--Leather shoes with or without laces are fine. Sport shoes, even made out of leather are inappropriate.
--Shoes should be in dark color, most likely black. Dark brown is also acceptable.
--Socks must coordinate with the shoes. Generally speaking, they should be plain black or plain dark brown depending on the color of the shoes.

For women

it is far more difficult for me to define what is appropriate. The following suggestions are by no means conclusive. I have to ask our female colleagues to use their common sense and good judgement. However, I would still like to provide some obvious guidelines as follows.

--Generally speaking tops with collars and sleeves (either long or short) are preferred.
--Long pants or skirts are fine as long as they are of proper length.
--Shorts, mid-length pants (七分裤), mini skirts are out of the question.
--Ornaments and accessories are certainly fine, but excessive use is discouraged.
--Jeans and pants with metal studs all over are not appropriate.
--Shoes should be appropriate for working environment. Sandals and dancing shoes are not appropriate.
--Excessive exposure of skin such as low cut blouses are not acceptable.