Abstract: The spatial patterns and structures of diameter grades of main arbor populations in Quercus mongolica forest by different aspects and times of forest conservation were studied. The results showed that Q. mongolica and Ulmus laciniata populations were aggregated distribution; they displayed different degrees of aggregation intensity during different times of forest conservation; aggregation intensity of populations inclined as a whole. During initial stages of forest conservation, the aggregation intensity of U. laciniata population was greater than that of Q. mongolica, and the aggregation intensity of U. laciniata population declined and diffused gradually with prolong times of forest conservation. The quantities of Q. mongolica, Tilia mandshurica and U. laciniata populations declined with raising diameter grades on 5 years forest conservation, which indicated they were evolving populations. After 16 years forest conservation, T. mandshurica and U. laciniata populations were retrogressive to some extent for greater coverage in arbor layer and they became associated species of Q. mongolica population. Q. mongolica population became dominant species as a result of 24 years forest conservation. Coverage of arbor layer increased more rapidly on positive slope then on negative slope with longer time of forest conservation. The decline phenomenon of T. mandshurica and U. laciniata populations manifested more clearly on positive slope than on negative slope with longer time of forest conservation.