Abstract:A mathematical model to caculate the economical service life of EQ-140logging truck is established by means of trial caculation. The result showsthat the economical service life is 3.7 years when the purchase cost of a newtruck is $50000 and the interest is 8%;and the averaye cost does not changeobviously when the service life is 3.7 to 5 years; the higher the purchase costof a new truck is, the longer the economical service life is When the purc-base cost between $40000 and $60000, the economical service life is from3.3 to 5.1 years.