Abstract: This paper deals with the numerical cladistic taxonomy. A method for constructing evolutionary tree (method of maximal same step length) is proposed in the applications and practice of cladistic taxonomy. Its algorithm runs as follows: 1) According to the order of evolution, characters are coded with nonnegative integers, producing the original data matrix. 2) Calculate the same step coefficients Sij (i≠j) by the formula (3) and form the coefficient matrix. 3. Find the maximal value S_pq of the same step coefficients in the coefficient matrix. 4) According to the maximal same step length S_pq, the most recent common ancestor CTU, of CTUp and CTUq can be determinated by (2). 5) draw the cladistic edges of cladogram representing the evolutionary relationship from OTUT to OTUp and OTUq. If the number of CTUs in the data matrix≤2, go to (2), otherwise stop. An example of 6 species from the family Campanulaceae is given for illustration (See Table 1). In general case, the evolutionary length of the cladogram obtained by this method is shorter than that by monothetic and other methods. Its algorithm is easily performed and is especially suitable for computerizing.

Key words: Cladistic classilfication, Numerical taxonomy