Cognitive Behavioral Therapy (CBT) is frequently used to attenuate the severity of positive schizophrenia symptoms; however, few studies have focused on attenuating negative symptoms. Recently, researchers have become interested in the effects of mindfulness-based intervention (MBI) on schizophrenia, but the lack of evidence-based results from random clinical trials (RCTs) has limited their effectiveness. Moreover, longitudinal data must be examined using appropriate study designs. We recruited 60 schizophrenia patients and randomly assigned them to an MBI or to a treatment-as-usual group. Negative symptoms, positive symptoms, mindfulness, and depression were assessed at baseline, post-course, and at a 3-month follow-up. Descriptive analysis and generalized estimating equations (GEEs) were used to examine the effects of MBI. We found that MBI mitigated the severity of negative symptoms and of general schizophrenic psychopathology except for the positive symptoms and for those of depression. Unexpectedly, we did not find long-term effect of mindfulness on negative symptoms. Larger sample sizes, long-term practical course, more rigorous study procedures, and a double-blind design should be considered in future studies.