No, not chosen yet.  Two flowers, Peony and Plum Blossom, both are popular in China and have been considered as candidates for national flower for some time but none has been selected as the national flower.

Ok i did a little googling and here's what I've found out:

The National Flower of the Republic of China was officially designated as the plum blossom by the Executive Yuan of the Republic of China on July 21, 1964.[1] The plum blossom, known as the meihua (Chinese: 梅花; pinyin: méihuā), is symbol for resilience and perseverance in the face of adversity, because plum blossoms often bloom most vibrantly even amidst the harsh winter snow.[2][3] The three stamens represents Dr. Sun Yat-sen's Three Principles of the People, while the five petals symbolize the five branches of the government: Executive Yuan, Legislative Yuan, Judicial Yuan, Examination Yuan and Control Yuan.[1][3] The flower has also been proposed to be one of the national flowers for the People's Republic of China.[4]

reference:
http://en.wikipedia.org/wiki/National_Flower_of_the_Republic_of_China