We compute all string tree level correlation functions of vertex operators in c<1 string theory. This is done by using the ring structure of the theory. In order to study the multicritical behaviour, we calculate the correlation functions after perturbation by physical vertex operators. We show that the (2k-1,2) models can be obtained from the (1,2) model and the minimal models can be obtained from the (1,p) model by perturbing the action by appropriate physical operators. Our results are consistent with known results from matrix models.